Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D1: Phase Transitions and Transport in Quantum Hall Superfluids
Sponsoring Units: DCMPChair: Allan MacDonald, University of Texas at Austin
Room: Spirit of Pittsburgh Ballroom A
Monday, March 16, 2009 2:30PM - 3:06PM |
D1.00001: Phase Diagram of Bilayer 2D Electron Systems at $\nu _{T}$ = 1 Invited Speaker: Bilayer 2D electron systems at total filling fraction $\nu _{T}$ = 1 and small interlayer spacing can support a strongly correlated phase which exhibits spontaneous interlayer phase coherence and may be described as an excitonic Bose condensate. We use electron interlayer tunnelling and transport to explore the phase diagram of bilayer 2D electron systems at $\nu _{T}$ = 1, and find that phase transitions between the excitonic $\nu _{T}$ = 1 phase and bilayer states which lack significant interlayer correlations can be induced in three different ways: by increasing the effective interlayer spacing, d/$\ell $, the temperature, T, or the charge imbalance, $\Delta \nu =\nu _{1}-\nu _{2}$. First, for the balanced ($\Delta \nu $ = 0) system we find that the amplitude of the resonant tunneling in the coherent $\nu _{T}$ = 1 phase obeys an empirical power law scaling versus d/$\ell $ at various T, and the layer separation where the tunneling disappears scales linearly with T. Our results [1] offer strong evidence that a finite temperature phase transition separates the balanced interlayer coherent phase from incoherent phases which lack strong interlayer correlations. Secondly, we observe [2] that close to the phase boundary the coherent $\nu _{T}$ = 1 phase can be absent at $\Delta \nu $ = 0, present at intermediate $\Delta \nu $, and absent again at large $\Delta \nu $, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. Lastly, at $\Delta \nu $ = 1/3 we report [2] the observation of a direct phase transition between the coherent $\nu _{T}$ = 1 bilayer integer quantum Hall phase and the pair of single layer fractional quantized Hall states at $\nu _{1}$ = 2/3 and $\nu _{2}$ = 1/3.\\[4pt] [1] A.R. Champagne, \textit{et al.,} \textit{Phys. Rev. Lett}. \textbf{100}, 096801 (2008).\\[0pt] [2] A.R. Champagne, \textit{et al}, \textit{Phys. Rev. B }\textbf{78}, 205310 (2008) [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D1.00002: $\nu=1/2+1/2$ Quantum Hall Bilayers Invited Speaker: Quantum Hall bilayer systems at filling fractions near $\nu=1/2+1/2$ undergo a transition from a compressible phase with strong intralayer correlation to an incompressible phase with strong interlayer correlations as the layer separation $d$ is reduced below some critical value. Deep in the intralayer phase (large separation) the system can be interpreted as a fluid of composite fermions (CFs), whereas deep in the interlayer phase (small separation) the system can be interpreted as a fluid of composite bosons (CBs). The focus of this paper is to understand the states that occur for intermediate layer separation by using trial variational wavefunctions. We consider two main classes of wavefunctions. In the first class, previously introduced in we consider interlayer BCS pairing of two independent CF liquids. We find that these wavefunctions are exceedingly good for $d \agt \ell_0$ with $\ell_0$ the magnetic length. The second class of wavefunctions naturally follows the reasoning of [2] and generalizes the idea of pairing wavefunctions by allowing the CFs also to be replaced continuously by CBs. This generalization allows us to construct exceedingly good wavefunctions for interlayer spacings of $d \alt \ell_0$, as well. The accuracy of the wavefunctions discussed in this work, compared with exact diagonalization, approaches that of the celebrated Laughlin wavefunction. More details can be found online in [3]. \\[4pt] [1] G. Moller, S. H. Simon, and E. Rezayi PRL {\bf 101}, 176803 (2008). \\[0pt] [2] S. H. Simon, E. Rezayi, and M. Milovanovic PRL {\bf 91}, 046803 (2003) \\[0pt] [3] G. Moller, S. H. Simon, and E. Rezayi, arXiv:0811.4116 [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D1.00003: Optical probes of excitonic phases in quantum Hall bilayers at $\nu_T$=1* Invited Speaker: In this talk we discuss our recent inelastic light scattering results that shed light on the interplay between incompressible and compressible quantum phases of electron bilayers at total filling factor $\nu_T$ = 1. In the regime of finite values of tunneling gaps, we observe a quantum phase transformation between composite fermion (CF) metal and incompressible excitonic states as the tunneling gap is reduced. We show that the transition becomes discontinuous (first-order) by impacts of different terms of the electron-electron interactions that prevail on weak residual disorder [1]. The evidence is based on precise determinations of the excitonic order parameter and of measurements of CF spin excitations by resonant inelastic light scattering close to the phase boundary [2,3]. While there is marked softening of low-lying excitations, our experiments underpin the roles of competing order parameters linked to quasi-particle correlations in removing the divergence of quantum fluctuations [4]. In the regime of vanishingly small tunneling gaps we show that the abrupt disappearing of CF spin excitations below the spin-wave indicates the emergence of the inter-layer correlated quantum Hall state in the vicinity of $\nu _T$ = 1 and when the temperature is lowered below a critical value [5]. Finally, the evolution of the spin-wave mode as a function of the Zeeman energy suggests the occurrence of a spin transition [5]. * Work done in collaboration with: B. Karmakar, A. Pinczuk, L.N. Pfeiffer, K.W. West.\\[4pt] [1] B. Karmakar, submitted; [2] S. Luin, et al. Phys. Rev. Lett. {\bf 94}, 146804 (2005); [3] B. Karmakar et al. Solid State Communications {\bf 143}, 499 (2007); [4] J. Schliemann, S. M. Girvin and A. H. MacDonald, Phys. Rev. Lett. {\bf 86}, 1849 (2001); [5] B. Karmakar et al. unpublished. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D1.00004: Theory of Activated Transport in Bilayer Quantum Hall Systems Invited Speaker: We analyze the transport properties of bilayer quantum Hall systems at total filling factor ? = 1 in drag geometries as a function of interlayer bias, in the limit where the disorder is sufficiently strong to unbind meron-antimeron pairs, the charged topological defects of the system. We compute the typical energy barrier for these objects to cross incompressible regions within the disordered system using a Hartree-Fock approach, and show how this leads to multiple activation energies when the system is biased. We then demonstrate using a bosonic Chern-Simons theory that in drag geometries, current in a single layer directly leads to forces on only two of the four types of merons, inducing dissipation only in the drive layer. Dissipation in the drag layer results from interactions among the merons, resulting in very different temperature dependences for the drag and drive layers. Connections with recent experiments will be discussed. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D1.00005: Spin-dependent phase diagram in bilayer 2D electron systems Invited Speaker: Bilayer electron systems with total filling $\nu=1$ involve rich physics arising from the interplay between the intralayer and interlayer interactions parameterized by the ratio between the interlayer distance $d$ and the magnetic length $\ell_{B}$. One key issue in this system is the nature of the phase transition that occurs when exploring the system between the two limits of weak and strong interlayer interactions, i.e., compressible Fermi-liquid states of composite fermions and an incompressible quantum Hall state. Here we report tilted-field experiments on a double quantum well with negligible tunneling that demonstrate that the spin degree of freedom plays a decisive role in the ground-state phase diagram of this system [1]. When the ratio $\eta$ of the Zeeman to Coulomb energies is enhanced by tilting the sample in a field by an angle $\theta$, we observe that the phase boundary located at $d/\ell_{B}=1.90$ for $\theta=0$ shifts to higher densities until it saturates at $d/\ell_{B}=2.33$ for $\theta \geq 60$ degree. The data thus establish a spin-dependent phase diagram as a function of $\eta$ and $d/\ell_{B}$. We model the energies of the competing phases treating the compressible state as nearly independent Fermi liquids of composite fermions. The excellent agreement between the model and experiment indicates that at small $\theta$ the compressible state is only partially polarized and its Zeeman-dependent energy is responsible for the observed shift of the phase boundary, with the saturation at large $\theta$ signaling the full polarization. This in turn implies that the intrinsic transition, expected for the ideal system without spin and intensively studied in theory, is preempted by a transition to a partially polarized compressible state in the standard experimental conditions and can only be revealed by suppressing the spin degree of freedom. Our results thus shed new light on previous experiments and show a way to investigate the intrinsic properties of the system. [1] P. Giudici \textit {et al}., Phys. Rev. Lett. \textbf{100}, 106803 (2008). [Preview Abstract] |
Session D2: HTSC: New Developments
Sponsoring Units: DCMPChair: Chandra Varma, University of California, Riverdside
Room: Spirit of Pittsburgh Ballroom BC
Monday, March 16, 2009 2:30PM - 3:06PM |
D2.00001: New magnetic neutron scattering results for the high-T$_{c}$ superconductors HgBa$_{2}$CuO$_{4+\delta }$and Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta }$ Invited Speaker: We have succeeded in growing sizable single crystals of HgBa$_{2}$CuO$_{4+\delta }$, the single-layer hole-doped compound with the highest superconducting transition temperature [1]. Careful characterization demonstrates the high quality of our crystals [2]. Using polarized neutron diffraction, we find an unusual magnetic order in the pseudogap phase [3]. Together with prior results for YBa$_{2}$Cu$_{3}$O$_{6+\delta }$, this observation constitutes a demonstration of the universal existence of such a state. Our inelastic neutron scattering measurements reveal that the antiferromagnetic resonance occurs at a rather high energy in HgBa$_{2}$CuO$_{4+\delta }$ and, hence, that the resonance energy is not universally related to T$_{c}$ [4]. Finally, our results for the antiferromagnetic excitations in Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta }$ provide new insight into magnetic energy scales and the resonance of the electron-doped side of the phase diagram [5]. \\[4pt] [1] X. Zhao et al., Adv. Mater. \textbf{18}, 3243 (2006). \\[0pt] [2] N. Barisic et al., Phys. Rev. B \textbf{78}, 054518 (2008). \\[0pt] [3] Y. Li et al., Nature \textbf{455}, 372 (2008). \\[0pt] [4] G. Yu et al., arXiv:0810.5759. \\[0pt] [5] G. Yu et al., arXiv:0803.3250. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D2.00002: Quantum critical fluctuations in cuprates and d-wave superconductivity through their coupling to fermions Invited Speaker: The phase diagram of the Cuprates is a collection of anomalies that has challenged our understanding of quantum many body physics. An organizing principle proposed to unify the experimental observations is the existence of a quantum critical point near optimal doping separating a phase which has broken time reversal and a renormalized Fermi liquid. I will discuss recent theoretical developments related to the nature of the fluctuations near the quantum critical point. The long wavelength theory of the time reversal violating state belongs to the dissipative 2DXY universality class. The fluctuation spectrum, at the quantum critical point, is local in space and power law in time, precisely of the form observed in the marginal fermi liquid phase near optimal doping. The fluctuations couple to the local angular momentum of the fermions to give a momentum dependence to the coupling which leads directly to pairing attraction in the d-wave channel. \newline Theory of superconductivity in the cuprates, Vivek Aji, Arkady Shekhter and Chandra Varma, arXiv:0807.3741 \newline Theory of the quantum critical fluctuations in cuprate superconductors, Vivek Aji and Chandra Varma, Physical Review Letters, \textbf{99}, 067003 (2007) [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D2.00003: Search for a thermodynamic evidence of a phase transition in the pseudogap state of YBa$_{2}$Cu$_{3}$O$_{6+x}$ Invited Speaker: Recent polarized neutrons diffraction experiments have evidenced a symmetry breaking in the underdoped phase of superconducting cuprates [1,2,3]. This symmetry breaking takes place below a temperature T$_{MAG}$ which increases when the number of charge carriers in the superconducting planes is decreased. We present here magnetic susceptibility measurements in YBa$_{2}$Cu$_{3}$O$_{6+x}$ ceramics. We have measured the magnetization of about twenty samples with different oxygen contents under 1T magnetic field. In some of them, we have observed an anomaly in the temperature derivative of the susceptibility at a temperature T$_{1}$ which seems to be in good agreement to the temperature T$_{MAG}$. We show here the resulting phase diagram for underdoped YBa$_{2}$Cu$_{3}$O$_{6+x}$. These findings raise the question of a thermodynamic evidence for a phase transition in relation to the symmetry breaking observed by neutrons.\\[4pt] [1] B. Fauque et al, Phys. Rev. Letters 96, 197001 (2006). \\[0pt] [2] H. A. Mook et al, Phys. Rev. B 78 ,020506(2008) \\[0pt] [3] Y. Li, Nature 455, 372 (2008) [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D2.00004: Competition between the pseudogap and superconductivity in cuprates Invited Speaker: The relationship between the pseudogap and superconductivity is one of the central issues in physics of cuprates. By studying the spectral weights associated with pseudogap and superconductivity by angle resolved photoemission spectroscopy (ARPES) we found that there is a direct correlation between the loss of the low energy spectral weight due to the opening of the pseudogap and a decrease of the spectral weight associated with superconductivity as a function of momentum and doping. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing in the region of momentum space, where the superconducting gap is largest. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D2.00005: Nanoscale Imaging of the Pair Formation in High-Temperature Superconductors Invited Speaker: In the quest for a microscopic theory for the superconductivity in cuprates, one hotly debated issue is the temperature at which Cooper pairs first form. Do pairs form at the critical temperature Tc or do they form at higher temperatures lacking phase rigidity? To answer the question, we have developed new techniques, based on the scanning tunneling microscope, to visualize the process of the pair formation on the atomic scale. The magnitude of the low-temperature superconducting gap measured in Bi-2212 shows a large nanoscale spatial variation. These superconducting gaps evolve smoothly with temperature and close locally over a range of temperatures above the superconducting transition temperature Tc [1]. Our results provide evidence that pairing first occurs in nanoscale regions above the bulk superconducting transition temperature. Using the ability to track the same atomic position while changing the temperature, we have examined the evolution of the electronic states from well below Tc to above the temperature at which the pairs first form. Our technique allows us to investigate another fundamental question on the pairing mechanism: Is pairing mediated by a bosonic excitation, as in conventional BCS superconductors, or is pairing with d-wave symmetry an unavoidable consequence of the strong Coulomb repulsion in these compounds? We quantitatively analyze the temperature evolution of the gap and the local electron-boson coupling for various atomic sites with different pairing strengths [2]. We observe that the gap magnitude variation is not determined by the electron-boson coupling but instead it is strongly correlated to variations present in the normal (ungapped) electronic states. \\[4pt] [1] Gomes KK, Pasupathy AN, Pushp A, Ono S, Ando Y, Yazdani A, Nature 447, 569 (2007). \\[0pt] [2] Pasupathy AN, Pushp A, Gomes KK, Parker CV, Wen J, Xu Z, Gu G, Ono S, Ando Y, Yazdani A, Science 320, 196 (2008). [Preview Abstract] |
Session D3: Pake, McGroddy, and Industrial Application of Physics Prizes, Adler Award
Sponsoring Units: FIAP DMPChair: Mark Lee, Sandia National Laboratories
Room: 301/302
Monday, March 16, 2009 2:30PM - 3:06PM |
D3.00001: George E. Pake Prize Talk: Kosterlitz-Thouless Transitions, Weak Localization, Aharonov-Bohm Oscillations, Heavy Fermions, Flux Lattice Melting, Casimir Forces and other Pleasures from a Career in Low Temperature Physics Invited Speaker: In my talk I will discuss and review a number of the memorable experiments that I have had the good fortune to be affiliated with over a thirty-five year career spent in low temperature physics. They range from the first observation of the Kosterlitz-Thouless Transition in superfluid helium films to mechanical measurements of the Casimir Force using a MEMS microbalance. These experiments were done at Cornell and Bell Labs. It has been my privilege to be affiliated with these two premier institutions which are among the very best places in the world to do low temperature physics. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D3.00002: Prize for Industrial Applications of Physics Talk: The Inverse Scattering Problem and the role of measurements in its solution Invited Speaker: The electromagnetic inverse scattering problem suggests that if a homogeneous and non-absorbing object be illuminated with a monochromatic light source and if the far field scattered light intensity is known at sufficient scattering angles, then, in principle, one could derive the dielectric structure of the scattering object. In general, this is an ill-posed problem and methods must be developed to regularize the search for unique solutions. An iterative procedure often begins with a model of the scattering object, solves the forward scattering problem using this model, and then compares these calculated results with the measured values. Key to any such solution is instrumentation capable of providing adequate data. To this end, the development of the first laser based absolute light scattering photometers is described together with their continuing evolution and some of the remarkable discoveries made with them. For particles much smaller than the wavelength of the incident light (e.g. macromolecules), the inverse scattering problems are easily solved. Among the many solutions derived with this instrumentation are the in situ structure of bacterial cells, new drug delivery mechanisms, the development of new vaccines and other biologicals, characterization of wines, the possibility of custom chemotherapy, development of new polymeric materials, identification of protein crystallization conditions, and a variety discoveries concerning protein interactions. A new form of the problem is described to address bioterrorist threats. Over the many years of development and refinement, one element stands out as essential for the successes that followed: the R and D teams were always directed and executed by physics trained theorists and experimentalists. 14 Ph. D. physicists each made his/her unique contribution to the development of these evolving instruments and the interpretation of their results. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D3.00003: James C. McGroddy Prize Talk: Metallic Glasses Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D3.00004: James C. McGroddy Prize Talk: Development and Applications of Bulk Metallic Glasses Invited Speaker: We realized, through detailed amorphous material investigations in the 1980's, that a number of metallic glassy alloys of multi-component La-, Mg- and Zr-based systems exhibit a large supercooled-liquid region prior to crystallization. The stabilization phenomenon of these supercooled liquid should enable us to fabricate, by slow cooling processes, bulk metallic glasses (BMGs) with critical diameters larger than several millimeters. Caltech's group also succeeded the fabrication of BMG in Zr-based alloy system in 1993. Since then, much attention has been paid to BMGs because of their novel characteristics in basic science and engineering aspects and new materials science and engineering fields have emerged for BMGs. Based on knowledge obtained thus far, we have successfully developed new BMGs with technologically-important transition metals, such as Zr-, Ti-, Fe-, Co-, Ni- and Cu-based alloys. Currently, the maximum diameter for glass formation reaches 30 mm for Zr- and Cu-based systems, 12 mm for Ti-based system, 18 mm for Fe-Co-based system and 20 mm for Ni-based system, even employing the copper mold casting technique. These large size BMGs possess nearly the same fundamental properties as those of the BMGs with smaller diameters. BMGs with diameters above 10 mm can be formed in Zr-Al-Ni-Cu system with Zr compositions higher than 65 at{\%} and they exhibit excellent properties, such as high Poisson's ratio, high ductility, high fracture toughness, high fatigue strength and high stability of mechanical properties to annealing-induced embrittlement. The new Ti-based BMGs without allergic and toxic elements should exhibit good compatibility to bio-tissues. Applications of BMGs in Fe-, Co-, Ti- and Zr-based systems have advanced many devices including the following; choke coil, power inductor, electro magnetic shielding, magnetic and position sensors, micro-geared motor, pressure sensor, Coriolis flowmeter, surface coating layer, precise polishing medium, magnetic and structural parts in electric magnetic control-type spring drive watches, medical operation instruments and so forth. A. Inoue, Acta Mater., 48(2000), 279-306. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D3.00005: David Adler Lectureship Award: n-point Correlation Functions in Heterogeneous Materials. Invited Speaker: The determination of the bulk transport, electromagnetic, mechanical, and optical properties of heterogeneous materials has a long and venerable history, attracting the attention of some of the luminaries of science, including Maxwell, Lord Rayleigh, and Einstein. The bulk properties can be shown to depend rigorously upon infinite sets of various $n$-point correlation functions. Many different types of correlation functions arise, depending on the physics of the problem. A unified approach to characterize the microstructure and bulk properties of a large class of disordered materials is developed [S. Torquato, {\it Random Heterogeneous Materials: Microstructure and Macroscopic Properties} (Springer-Verlag, New York, 2002)]. This is accomplished via a {\it canonical} $n$-point function $H_n$ from which one can derive exact analytical expressions for any microstructural function of interest. This microstructural information can then be used to estimate accurately the bulk properties of the material. Unlike homogeneous materials, seemingly different bulk properties (e.g., transport and mechanical properties) of a heterogeneous material can be linked to one another because of the common microstructure that they share. Such cross-property relations can be used to estimate one property given a measurement of another. A recently identified {\it decorrelation principle}, roughly speaking, refers to the phenomenon that unconstrained correlations that exist in low-dimensional disordered materials vanish as the space dimension becomes large. Among other results, this implies that in sufficiently high dimensions the densest spheres packings may be disordered (rather than ordered) [S. Torquato and F. H. Stillinger, ``New Conjectural Lower Bounds on the Optimal Density of Sphere Packings," {\it Experimental Mathematics}, {\bf 15}, 307 (2006)]. [Preview Abstract] |
Session D4: Spin Qubits in Quantum Dots
Sponsoring Units: GQI DCMPChair: Jason Petta, Princeton University
Room: 306/307
Monday, March 16, 2009 2:30PM - 3:06PM |
D4.00001: Singlet-triplet dynamics in double quantum dots probed with single-shot readout Invited Speaker: We report single-shot readout of a two-electron spin qubit in a GaAs double quantum dot. The readout scheme allows repeated single quantum measurements with a readout fidelity above $90\%$, extracted from a simple model of the measurement outcome. In contrast to measurements on single spins, this scheme does not rely on tunneling of electrons out of the quantum dot or on high magnetic fields, which are incompatible with the operating requirements of the qubit. The spin state is mapped to a charge state, which is subsequently measured by a rapidly switched quantum point contact~(QPC). Single-shot readout is used to observe the precession of the electron spin qubit in the effective magnetic field due to the hyperfine interaction with the GaAs nuclei. All measurements are taken within the nuclear correlation time and the evolution of the nuclear spin bath is monitored continuously on a ms time scale. Finally the precession of the qubit is compared to the driven dynamics of the electron spin state at the resonance between singlet and a triplet with total spin one. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D4.00002: Locking electron spins into resonance by electron-nuclear feedback Invited Speaker: All basic building blocks for spin-based quantum information processing using electron spins in GaAs quantum dots have recently been realized. Recent experiments have shown single-shot read-out of an individual spin [1], the implementation of the SWAP gate [2] and (magnetically induced) coherent single electron spin rotations [3]. However, the main drawback of using electron spins in a GaAs environment is the short spin coherence time, which is measured to be in the nanosecond range [2,4]. The source of this fast decoherence is the hyperfine interaction of the localized electron spin with the randomly fluctuating nuclear spins of the host lattice. The fluctuations of the nuclear spins have to be reduced to extend the electron spin coherence time. We therefore study the electron-nuclear spin interaction and use magnetically driven spin resonance to control the electron spin and indirectly manipulate the nuclear spins. We apply continuous microwave excitation to the electron spin and observe strong electron-nuclear feedback. One experimental signature of this feedback is the locking of the electron spin system into resonance with the microwaves. Once the electron spin is locked into resonance, this resonance condition remains fullfilled even when the external magnetic field or the microwave frequency is changed. This is due to dynamically build up nuclear polarizations (up to 500 mT) which generally counteract the external magnetic field. Locking of the electron spin system into resonance might indicate that the nuclear polarization exhibits stable configurations where fluctuations of the nuclear distribution are reduced [5]. \\[4pt] References \\[0pt] [1] J. M. Elzerman et al. , \textit{Nature} \textbf{430}, 431 (2004) \\[0pt] [2]. J. R. Petta et al., \textit{Science} \textbf{309}, 2180 (2005). \\[0pt] [3] F. H. L. Koppens et al., \textit{Nature }\textbf{442}, 766 (2006). \\[0pt] [4] F. H. L. Koppens et al., \textit{Phys. Rev. Lett.} \textbf{100}, 236802 (2008). \\[0pt] [5] J. Danon and Yu. V. Nazarov, \textit{private communication}. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D4.00003: Decoherence mechanisms for electron and hole spins in quantum dots Invited Speaker: One major obstacle to the realization of electron-spin qubits is decoherence with a random environment. While relaxation ($T_1$) processes are dominated in these systems by spin-orbit coupling and phonon emission, much faster dephasing processes are determined by coupling to an uncontrolled environment of nuclear spins. I will review work on electron-spin decoherence due to nuclear spins [1] and how to control this decoherence through a sequence of measurements performed on the nuclear-spin system [2,3]. This talk will then focus on coherence properties of \emph{hole}, rather than electron spins. Remarkably, in contrast to statements frequently made in the literature, we have found that the coupling of hole spins to nuclei can be appreciable [4] (comparable to that for electrons). However, in a two-dimensional quantum dot, the hole-nuclear spin coupling takes on an Ising-like form, which may allow for substantially longer coherence times than for electron spins.\\[4pt] [1] W. A. Coish, J. Fischer and D. Loss, Phys. Rev. B 77, 125329 (2008)\\[0pt] [2] D. Klauser, W. A. Coish, and D. Loss, Phys. Rev. B 73, 205302 (2006)\\[0pt] [3] D. Klauser, W. A. Coish, and D. Loss, Phys. Rev. B 78, 205301 (2008)\\[0pt] [4] J. Fischer, W. A. Coish, D. V. Bulaev, and D. Loss, Phys. Rev. B 78, 155329 (2008) [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D4.00004: Experiments on hole spins in quantum dots Invited Speaker: A single electron in a nano-sized quantum dot is so strongly quantized that the interaction with the phonons is highly suppressed. This leads to potentially long spin dephasing times, ideal for applications in quantum information. However, the hyperfine interaction, the interaction of the electron spin with the spins of the host nuclei, leads to a rapid loss of spin coherence and this presently represents the largest stumbling block in quantum dot spin physics. An alternative to an electron spin is a hole spin. The p-like atomic part of the hole wave function conveniently goes to zero at the locations of the atomic nuclei, removing the contact part of the hyperfine interaction. Could it be the case that a heavy hole spin is more coherent than an electron spin in a self-assembled quantum dot? Optical experiments will be presented on single hole spins, exploring the spin relaxation time with an optical pumping experiment and the spin dephasing time with coherent population trapping. The results are very encouraging: both the hole spin T1 and T2* times are surprisingly large. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D4.00005: Dynamic nuclear polarization with single electron spins Invited Speaker: Hyperfine interactions limit electron spin coherence times in GaAs quantum dots. By separating a spin singlet state on a chip, we measure an ensemble averaged spin dephasing time $T_2^*$ of 10 ns, limited by the contact hyperfine interaction with the GaAs host nuclei\footnote{J. R. Petta \textit{et al.}, Science $\bf{309}$, 2180 (2005).}. We use electrical control of the exchange interaction to drive coherent spin rotations. Exchange driven spin rotations are used to implement a ``singlet-triplet spin echo'' pulse sequence, which leads to a spin coherence time, $T_2$, exceeding 1 microsecond. We show that nuclear spins can be polarized by controlling two-electron spin states near the anti-crossing of the singlet (S) and triplet ($T_+$). An initialized S state is cyclically brought into resonance with the $T_+$ state, where hyperfine fields drive rapid rotations between S and $T_+$, ``flipping'' an electron spin and ``flopping'' a nuclear spin\footnote{J. R. Petta, J. M. Taylor \textit{et al.}, Phys. Rev. Lett. $\bf{100}$, 067601 (2008).}. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage. [Preview Abstract] |
Session D5: Origins of Silicon Valley
Sponsoring Units: FHPChair: Gloria Lubkin, American Institute of Physics
Room: 401/402
Monday, March 16, 2009 2:30PM - 3:06PM |
D5.00001: Prehistory of Silicon Valley, from 1910 to 1965 Invited Speaker: The term ``Silicon Valley'' was coined in 1971, some six decades after the emergence of the San Francisco Bay Area as a center of innovation and invention in the fields of radio and electronics. The geographical position of San Francisco with respect to continental and Pacific transportation and communication needs; the growth of West Coast universities, markets, and population; the importation of talent from the East; innovative industrial and business methods--all these provided a thriving center of instrumentation, electronics, avionics, and high energy physics when Silicon arrived in the ``Valley of the Heart's Delight.'' [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D5.00002: W. W. Hansen, Microwave Physics, and Silicon Valley Invited Speaker: The Stanford physicist W. W. Hansen (b. 1909, AB '29 and PhD '32, MIT post-doc 1933-4, Prof. physics '35-'49, d. 1949) played a seminal role in the development of microwave electronics. His contributions underlay Silicon Valley's postwar ``microwave'' phase, when numerous companies, acknowledging their unique scientific debt to Hansen, flourished around Stanford University. As had the prewar ``radio'' companies, they furthered the regional entrepreneurial culture and prepared the ground for the later semiconductor and computer developments we know as Silicon Valley. In the 1930's, Hansen invented the cavity resonator. He applied this to his concept of the radio-frequency (RF) linear accelerator and, with the Varian brothers, to the invention of the klystron, which made microwave radar practical. As WWII loomed, Hansen was asked to lecture on microwaves to the physicists recruited to the MIT Radiation Laboratory. Hansen's ``Notes on Microwaves,'' the Rad Lab ``bible'' on the subject, had a seminal impact on subsequent works, including the Rad Lab Series. Because of Hansen's failing health, his postwar work, and MIT-Stanford rivalries, the Notes were never published, languishing as an underground classic. I have located remaining copies, and will publish the Notes with a biography honoring the centenary of Hansen's birth. After the war, Hansen founded Stanford's Microwave Laboratory to develop powerful klystrons and linear accelerators. He collaborated with Felix Bloch in the discovery of nuclear magnetic resonance. Hansen experienced first-hand Stanford's evolution from its depression-era physics department to corporate, then government funding. Hansen's brilliant career was cut short by his death in 1949, after his induction in the National Academy of Sciences. His ideas were carried on in Stanford's two-mile long linear accelerator and the development of Silicon Valley. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D5.00003: From Bell Labs to Silicon Valley: A Saga of Technology Transfer, 1954-1961 Invited Speaker: Although Bell Telephone Laboratories invented the transistor and developed most of the associated semiconductor technology, the integrated circuit or microchip emerged elsewhere--at Texas Instruments and Fairchild Semiconductor Company. I recount how the silicon technology required to make microchips possible was first developed at Bell Labs in the mid-1950s. Much of it reached the San Francisco Bay Area when transistor pioneer William Shockley left Bell Labs in 1955 to establish the Shockley Semiconductor Laboratory in Mountain View, hiring a team of engineers and scientists to develop and manufacture transistors and related semiconductor devices. But eight of them--including Gordon Moore and Robert Noyce, eventually the co-founders of Intel--resigned en masse in September 1957 to start Fairchild, bringing with them the scientific and technological expertise they had acquired and further developed at Shockley's firm. This event marked the birth of Silicon Valley, both technologically and culturally. By March 1961 the company was marketing its Micrologic integrated circuits, the first commercial silicon microchips, based on the planar processing technique developed at Fairchild by Jean Hoerni. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D5.00004: The Origins and Development of the Silicon Valley Startup Model Invited Speaker: |
Session D6: Predictive Materials Design for Alternative Energy Storage
Sponsoring Units: DCOMPChair: Zhenyu Zhang, Oak Ridge National Laboratory
Room: 406
Monday, March 16, 2009 2:30PM - 3:06PM |
D6.00001: First-Principles Studies of Phase Stability and Reaction Dynamics in Complex Metal Hydrides Invited Speaker: Complex metal hydrides are believed to be one of the promising materials for developing hydrogen storage systems that can operate under desirable conditions. At the same time, these are also a class of materials that exhibit intriguing properties. We have applied state-of-the-art computational techniques to study the structural, dynamic, and electronic properties of these materials. This talk will focus on the critical role played by the Ti catalyst in helping hydrogen cycling in the alanates, which remains a challenging topic for this hydrogen storage material. We have performed a series of calculations to address the hydrogen interaction on the aluminum surface in the presence of the Ti ``dopant,'' focusing on the effect of near-surface alloying on the Al(100) surface. It is found that Ti occupies subsurface sites near the Al surface. This subsurface Ti arrangement not only enhances H binding with the Al surface layer, but also improves H mobility on the surface. Based on existing experimental data and our preliminary results, we propose a model in which the catalyst does not enter the bulk, but facilitates hydrogen dissociation-recombination near the surface. In the dehydrogenation cycle, the catalyst kinetically facilitates the release and decomposition of AlH$_3$ from the solid-state alanate. In the hydrogenation cycle, the catalyst helps the adsorption of hydrogen and the formation of AlH$_3$ oligomers on Al surfaces. The implication of Ti as a catalyst for the hydrogenation reactions will be discussed. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D6.00002: Energy Storage in Nanostructured Materials Invited Speaker: Renewably produced energy by solar and wind technologies should be stored properly for practical use because of their intermittent generation of electricity. The energy can be stored in materials in forms of chemical, electrical, or thermal energies. The current energy-storage materials technologies, however, suffer from their inevitable low energy densities, compared to liquid fuels such as gasoline and ethanol, and thus end up to high cost due to material limitation. In order to overcome the fundamental limit, many scientists and researchers have studied nanostructured materials with more surface areas, tunable storage mechanisms, and better kinetic processes. Because electronic and mechanical properties of nanostructured materials are simply not a miniature of their bulk counterparts, a careful material design is required based on microscopic understanding of the energy storing process. In this talk, I will discuss our recent theoretical efforts and development to understand energy storage mechanisms in nanostructured materials for hydrogen, battery, and electrochemical capacitor applications. We have pioneered dihydrogen adsorption in nanostructured materials with the Kubas coordination [1-3] and lately developed efficient van der Waals potentials within the density functional theory approach [4]. Also very recently we have unraveled reversible lithium intercalation mechanisms in MoO$_{3}$ nanoparticles for Li-ion battery electrodes [5], and been developing a microscopic theory of electrochemical and capacitive energy storage. \\[4pt] [1] Y. Zhao et al., Phys. Rev. Lett. \textbf{94}, 155504 (2005) \\[0pt] [2] Y.-H. Kim et al., Phys. Rev. Lett. \textbf{96}, 016102 (2006) \\[0pt] [3] Y. Y. Sun, Y.-H. Kim, and S. B. Zhang, J. Am. Chem. Soc. \textbf{129}, 12606 (2007) \\[0pt] [4] Y. Y. Sun, Y.-H. Kim, K. Lee, and S. B. Zhang, J. Chem. Phys. \textbf{129}, 154102 (2008) \\[0pt] [5] S.-H. Lee et al., Adv. Mater. \textbf{20}, 3627 (2008) [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D6.00003: Nanomaterials for Hydrogen Storage Invited Speaker: The success of a hydrogen economy critically rests on our ability to find materials that can store hydrogen with large gravimetric and volumetric densities and operate at near ambient conditions. To meet the large gravimetric density requirement, the storage materials must be lighter than Al. Unfortunately, in these light materials hydrogen is bound either too strongly or too weakly, thus leading to poor thermodynamics. I will discuss how the chemistry of these elements can be manipulated at the nanoscale so that hydrogen can be stored in quasi-molecular form with binding energies that are appropriate for near ambient conditions. Examples will include functionalized carbon fullerenes and nanotubes and doped AlN nanostructures. Using carbon nanostructures as catalysts I will demonstrate unambiguously the dehydrogentation mechanism of sodium alanate. A cluster perspective of the intermediate phases in the dehydrogenation of borohydrides will also be presented. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D6.00004: Fundamental design of hydrogen storage structures and systems. Invited Speaker: Fundamental simulations of hydrogen interactions with host structures offer indispensable insights to the understanding and design of hydrogen storage materials for practical applications. First-principles approaches were applied to selected materials of high promise, e.g., doped/defective carbon, doped hydrides and metal/amine complexes. Several candidates show large capacities for hydrogen -- over the 6-9 mass-percentage threshold considered for applications. Recent progress on carbon nanostructures show the importance of defects and doping on extra hydrogen uptake, and a small change of C-C interspacing on the mode-switching of hydrogen sorption. Work on the molecular analogues of the basic structural unit of boron-nitride indicates that transition metal (TM) atom doping can boost both the gravimetric and thermodynamic capacities of hydrogen in these materials. The H2 binding to the TM dopants is Kubas-like in nature, though the maximum binding capacity at the TM doped sites does not follow the 18-electron rule. Progress in the Li-N-H system shows that the N-Li bond is weaker than one of the N-H bonds in LiNH2, and consequently LiNH2 can dissociate into: Li+ and (NH2)-, or (LiNH)- and H+. Hence, NH3 may evolve as a transient gas, if it is not sufficiently captured by a reactive component, e.g. LiH. Molecular dynamics calculations indicate that hydrogen deliveries are possible close to fuel-cell operation conditions. Comparison is also made with experiment where possible. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D6.00005: First principles design of electric-field-assisted high capacity hydrogen storage media Invited Speaker: Hydrogen has been viewed as a highly appealing energy carrier for renewable energy. To achieve economic feasibility hydrogen storage materials with high gravimetric and volumetric densities must be developed. However, no materials so far satisfy the essential criteria for economically feasible hydrogen storage. Therefore, there are necessities of breakthrough ideas and methods for developing new materials. In this talk, I will discuss the novel idea of electric-field-assisted hydrogen storage in nanostructures. Its central ingredient is to create high and strongly delocalized electric fields that are strong enough to attract hydrogen through polarization. Using quantum mechanical first-principles calculations, it has been shown that high electric fields can be easily established in a region close to the surface of nanostructures by electronic doping [1] or in charge compensated ways. The charging idea and its underlying physical mechanism can be generalized to many other related nanoscale materials that are of interest for hydrogen storage, as exemplified by alkaline-earth-metal coated carbon nanostructures [2] and charge transferred organic crystals [3]. \\[4pt] [1] M. Yoon, S. Yang, E. Wang, and Z. Zhang, Nano Lett. \textbf{7}, 2578 (2007).\\[0pt] [2] M. Yoon, S. Yang, C. Hicke, E. Wang, D. Geohegan, and Z. Zhang, Phys. Rev. Lett. \textbf{100}, 206806 (2008).\\[0pt] [3] M. Yoon and M. Scheffler (in preparation). [Preview Abstract] |
Session D7: Rare Events in Physics and Population Dynamics
Sponsoring Units: GSNPChair: Beate Schmittmann, Virginia Polytechnic Institute and State University
Room: 407
Monday, March 16, 2009 2:30PM - 3:06PM |
D7.00001: Transitions in the Kramers escape rate in classical and quantum field theories Invited Speaker: Small random fluctuations, either of thermal or quantum origin, are the cause of many important and interesting physical phenomena. These include chemical reactions, nucleation in phase transitions (i.e., the formation of a droplet of one phase within another phase), and the formation of unusual spatially localized states in various condensed matter systems. In all of these, random fluctuations (or ``noise''), no matter how small, eventually drives a physical system from one stable state to another. We discuss how in some classical systems thermally activated hopping over a barrier undergoes a transition as an external parameter such as system size or external field is varied. Its features are similar to those arising when classical activation over a barrier crosses over to quantum tunneling through that barrier as temperature is lowered. This crossover has some (but not all of the) features of a second-order phase transition. We also discuss two timely applications from mesoscopic physics: thermally induced breakup of monovalent metallic nanowires, and stochastic reversal of magnetization in thin ferromagnetic annuli. Each are of interest both from the point of view of fundamental physics and for potential technological applications. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D7.00002: Stochastic predator-prey models: spatial variability enhances species fitness Invited Speaker: It is now well understood that including spatial structure and stochastic noise in models for predator-prey interactions invalidates the classical deterministic Lotka-Volterra picture of neutral population cycles. In contrast, stochastic models yield long-lived, but ultimately decaying erratic population oscillations, which can be understood through a resonant amplification mechanism for density fluctuations. Simulations of spatial stochastic predator-prey systems yield striking complex spatio-temporal structures. These spreading activity fronts induce persistent correlations between predators and prey. Here, we address the influence of spatially varying reaction rates on a stochastic two-species Lotka-Volterra lattice model. The effects of this quenched randomness on population densities, transient oscillations, spatial correlations, and invasion fronts are investigated through Monte Carlo simulations. We find that spatial variability in the predation rate results in more localized activity patches. Population fluctuations in rare favorable regions in turn cause a remarkable increase in the asymptotic population densities of both predators and prey, and also lead to accelerated front propagation. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D7.00003: Fluctuations in epidemic modeling - disease extinction and control Invited Speaker: The analysis of infectious disease fluctuations has recently seen an increasing rise in the use of new tools and models from stochastic dynamics and statistical physics. Examples arise in modeling fluctuations of multi-strain diseases, in modeling adaptive social behavior and its impact on disease fluctuations, and in the analysis of disease extinction in finite population models. Proper stochastic model reduction [1] allows one to predict unobserved fluctuations from observed data in multi-strain models [2]. Degree alteration and power law behavior is predicted in adaptive network epidemic models [3,4]. And extinction rates derived from large fluctuation theory exhibit scaling with respect to distance to the bifurcation point of disease onset with an unusual exponent [5]. In addition to outbreak prediction, another main goal of epidemic modeling is one of eliminating the disease to extinction through various control mechanisms, such as vaccine implementation or quarantine. In this talk, a description will be presented of the fluctuational behavior of several epidemic models and their extinction rates. A general framework and analysis of the effect of non-Gaussian control actuations which enhance the rate to disease extinction will be described. In particular, in it is shown that even in the presence of a small Poisson distributed vaccination program, there is an exponentially enhanced rate to disease extinction. These ideas may lead to improved methods of controlling disease where random vaccinations are prevalent. \\[4pt] Recent papers:\\[0pt] [1] E. Forgoston and I. B. Schwartz, ``Escape Rates in a Stochastic Environment with Multiple Scales,'' arXiv:0809.1345 2008.\\[0pt] [2] L. B. Shaw, L. Billings, I. B. Schwartz, ``Using dimension reduction to improve outbreak predictability of multi-strain diseases,'' J. Math. Bio. {\bf 55}, 1 2007.\\[0pt] [3] L. B. Shaw and I. B. Schwartz, ``Fluctuating epidemics on adaptive networks,'' Physical Review E {\bf 77}, 066101 2008.\\[0pt] [4] L. B. Shaw and I. B. Schwartz, ``Noise induced dynamics in adaptivenetworks with applications to epidemiology,'' arXiv:0807.3455 2008.\\[0pt] [5] M. I. Dykman, I. B. Schwartz, A. S. Landsman, ``Disease Extinction in the Presence of Random Vaccination,'' Phys. Rev. Letts. {\bf 101}, 078101 2008. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D7.00004: Forecasting fluctuating outbreaks in seasonally driven epidemics Invited Speaker: Seasonality is a driving force that has major impact on the spatio-temporal dynamics of natural systems and their populations. This is especially true for the transmission of common infectious diseases such as influenza, measles, chickenpox, and pertussis. Here we gain new insights into the nonlinear dynamics of recurrent diseases through the analysis of the classical seasonally forced SIR epidemic model. Despite many efforts over the last decades, it has been difficult to gain general analytical insights because of the complex synchronization effects that can evolve between the external forcing and the model's natural oscillations. The analysis advanced here attempts to make progress in this direction by focusing on the dynamics of ``skips'' where we identify and predict years in which the epidemic is absent rather than outbreak years. Skipping events are intrinsic to the forced SIR model when parameterised in the chaotic regime. In fact, it is difficult if not impossible to locate realistic chaotic parameter regimes in which outbreaks occur regularly each year. This contrasts with the well known Rossler oscillator whose outbreaks recur regularly but whose amplitude vary chaotically in time (Uniform Phase Chaotic Amplitude oscillations). The goal of the present study is to develop a ``language of skips'' that makes it possible to predict under what conditions the next outbreak is likely to occur, and how many ``skips'' might be expected after any given outbreak. We identify a new threshold effect and give clear analytical conditions that allow accurate predictions. Moreover, the time of occurrence (i.e., phase) of an outbreak proves to be a useful new parameter that carries important epidemiological information. In forced systems, seasonal changes can prevent late-initiating outbreaks (i.e., having high phase) from running to completion. These principles yield forecasting tools that should have relevance for the study of newly emerging and reemerging diseases. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D7.00005: Noise-activated switching and signal amplification in nonlinear resonators, from nanomechanical beams to superconducting striplines Invited Speaker: A driven nonlinear system operating close to bifurcation, namely, close to transition between different stability zones, is extremely sensitive to external perturbations. This behavior can be exploited for amplifying small signals, and also for noise reduction (squeezing). We experimentally demonstrate these effects using two classes of systems, namely, nanomechanical resonators in the form of doubly clamped beams, and electromagnetic resonators made of superconducting striplines. While a bifurcation between monostable and bistable zones is employed for the first class of resonators, a bifurcation between monostable and astable zones is employed for the second one. In both cases we observe extremely high gain and very strong noise squeezing as we approach bifurcation. While the Duffing-like nonlinearity of the mechanical beams is well understood, the piecewise-linear behavior exhibited by the superconducting stripline resonators is yet not fully accountable. We provide theoretical evidence to support our hypothesis that the underlying mechanism responsible for the observed piecewise-linear behavior is thermal instability in a narrow stripline section (a microbridge), which is integrated into the resonator. A simple theoretical model predicts a rich variety of dynamical effects, including self-sustained oscillations, stochastic resonance, and intermittency between different steadystate and limit-cycle solutions. These effects are experimentally observed by tuning the system close to the zone of astability, where no steadystate response exists. A comparison with theory yields partial agreement. Moreover, in more recent experiments we study a new configuration in which the microbridge is replaced by a superconducting interference device (SQUID) in the form of a loop containing two microbridges. Our preliminary experimental results show that self-sustained oscillations occur also in this configuration. Moreover, the frequency and lineshape of these oscillations exhibit periodicity as a function of externally applied magnetic flux. Further work is needed to theoretically account for the observed behavior. [Preview Abstract] |
Session D8: Unconventional Spin Torques
Sponsoring Units: GMAGChair: Shufeng Zhang, University of Missouri
Room: 414/415
Monday, March 16, 2009 2:30PM - 3:06PM |
D8.00001: Thermal Spin Transfer Torques Invited Speaker: The coupling between spin and charge in electronic transport is studied in the field of spintronics. Heat currents are coupled to both charge and spin currents as well [1]. This extension of spintronics to what may be called ``spin caloritronics'' recently enjoys renewed attention [2]. The spin-transfer torque associated with electric currents can excite magnetizations in nanostructures, switching magnetic configuration in spin valves and move domain walls in magnetic wires when exceeding critical values of the order of $10^7$Acm$^{-2}$ [3]. Also heat currents transfer spin angular momentum [4], either intrinsically or via the thermoelectric generation of particle spin currents. We predict that temperature differences of the order of 100 K over typical metallic nanostructures cause effects equivalent to the critical charge current densities. In this talk I will give a brief review of various aspects of spin caloritronics with emphasis on thermal spin transfer torques. This work has been carried out in collaboration with Moosa Hatami, Qinfang Zhang, Paul Kelly, Hans Joakim Skadsem, Arne Brataas and Sadamichi Maekawa. \\[4pt] [1] M. Johnson and R.H. Silsbee, Phys. Rev. B 35, 4959 (1987).\\[0pt] [2] International Workshop on Spin Caloritronics, Lorentz Center of Leiden University, 9-13 February 2009, http://www.lorentzcenter.nl/lc/web/2009/323/info.php3?wsid=323\\[0pt] [3] D. C. Ralph and M. D. Stiles, J. Magn. Magn. Mater. 320, 1190 (2008).\\[0pt] [4] M. Hatami, G.E.W. Bauer, Q. Zhang, and P.J. Kelly, Phys. Rev. Lett. 99, 066603 (2007). [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D8.00002: Nonequilibrium intrinsic spin torque in a single nanomagnet Invited Speaker: The spin transfer torque usually observed in metallic and tunneling spin-valves, as well as magnetic domain walls, comes from the transfer of the transverse spin-current of conduction electrons to the magnetization [1]. Therefore, it requires both a non collinear configuration of the magnetic structure (or inhomogeneous magnetic texture in the case of domain walls) and magneto-resistive effects. However, a number of magnetic systems show magneto-resistive effects in a single magnetic layer, such as anisotropic magnetoresistance (AMR) [2]. In the presence of spin-orbit interaction (SOI) the electron scattering depends on the magnetization direction. Recent theoretical studies suggest that in such systems, a transverse component of the spin density builds up, due to the spin-dependent scattering introduced by the spin-orbit coupling. Consequently, a transverse spin density arises from intrinsic properties of the band structure without the need of non-collinear magnetization texture. In the case of a single ferromagnet with spin-orbit interaction, the exchange interaction between the accumulated spin and the magnetization gives rise a spin torque on the magnetization. We show that this torque can be used to control the magnetization direction injecting current densities as low as 10\^{}5-10\^{}6 A/cm\^{}2, comparable or lower than the spin transfer effect. We first study the general case of a single ferromagnetic layer with spin-orbit interaction and then focus on the cases of effective Hamiltonians, such as Rashba and Dresselhaus SOI, as well as Luttinger hole systems. We discuss the relation between the spin torque and the spatial inversion symmetry of various forms of spin-orbit couplings and compare this spin torque with the conventional spin transfer torque. We finally discuss several magnetic systems for possible experimental realization. This work was done in collaboration with Shufeng Zhang. [1] ] J.C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353 (1996). [2] T.R. McGuire and R.I. Potter, IEEE Trans. Mag. 11, 1018 (1975). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D8.00003: Spin transfer torques in antiferromagnets and magnetic semiconductors Invited Speaker: Spin transfer torques (STT) in ferromagnetic metals can be understood in terms of conservation of total spin, allowing a simple evaluation and interpretation of these torques in terms of spin currents. STTs also occur in antiferromagnets, which have no net spin and different symmetries than ferromagnets, resulting in qualitatively different torques. We consider a structure with a compensated antiferromagnetic layer and a ferromagnetic layer. We find a STT on both layers, which vanishes when the layers' order parameters are either collinear or perpendicular. This torque can drive the magnetization of a thin film ferromagnet to be perpendicular to the easy plane. In dilute magnetic semiconductors, strong spin orbit coupling in the semiconductor host implies that spin is not even approximately conserved, requiring modifications of the microscopic calculation of the STT. We describe these modifications and present results from first principles calculations of STT in GaMnAs. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D8.00004: Spin-Transfer-Torques at a Ferromagnet/Antiferromagnet Interface Invited Speaker: Spintronics in ferromagnetic systems is built on a complementary set of phenomena in which the magnetic configuration of the system influences its transport properties and vice versa. Giant magnetoresistance (GMR) [1] and spin- transfer-torque (STT) [2] phenomena are typical examples of such interconnections. Recently, MacDonald and co-workers [3] predicted that corresponding effects ought to occur in systems where ferromagnetic (F) components are replaced by antiferromagnets (AFM). I will present our experimental search for these new AFM effects which may potentially lead to a new all-antiferromagnetic spintronics where antiferromagnets are used in place of ferromagnets. In particular I will focus on our experiments with exchange-biased spin valves [4] where extreme current densities were found to affect the exchange bias at F/AFM interface [5-7]. As exchange bias is known to be associated with interfacial AFM magnetic moments, our observation can be taken as the first evidence of STT effect in AFM materials. \\[4pt] [1] M. N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988); G. Binasch et al., Phys. Rev. B 39, 4828 (1989). \\[0pt] [2] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, J. Appl. Phys. 81, 4880 (1997); M. Tsoi et al., Phys. Rev. Lett. 80, 4281 (1998). \\[0pt] [3] A. S. N\'u\~nez et al., Phys. Rev. B 73, 214426 (2006); \\[0pt] [4] Z. Wei et al., Phys. Rev. Lett. 98, 116603 (2007). \\[0pt] [5] S. Urazhdin and N. Anthony, Phys. Rev. Lett. 99, 046602 (2007). \\[0pt] [6] X-L.Tang et al., Appl. Phys. Lett. 91, 122504 (2007). \\[0pt] [7] N. V. Dai et al., Phys. Rev. B77, 132406 (2008). [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D8.00005: Effects of polarizer dynamics on current-induced behaviors in magnetic multilayer nanopillars Invited Speaker: Magnetic nanodevices usually include two magnetic layers - a polarizing ``fixed'' layer, and a ``free'' layer, whose roles are determined by their relative thicknesses. I will describe our measurements of spin transfer in nanopillars with similar thicknesses of the ``polarizer'' and the ``free'' layer. In the first sample type, both layers were patterned into similar lateral dimensions. Spectroscopic measurements of current-induced dynamics showed incoherent bipolar excitations. Thermally-activated reversal statistics exhibited dependencies on magnetic field and applied current dramatically different from the ``standard'' samples with a thick polarizing layer. I will also discuss our results for samples in which only the free layer was patterned into a nanopillar, while the polarizing layer was left extended with dimensions of several micrometers. These samples exhibit coherent precession of only the extended layer, only the polarizer, or both, depending on the relative thicknesses of the two layers. The transition between the ``free''-like and ``fixed''-like behaviors of each layer occured over a small range of thickness. I will show that current-induced behaviors of our samples can be understood in terms of the dynamical coupling between ferromagnets induced by spin transfer. This coupling can result suppression of the current-induced precession, incoherent dynamics, or for certain geometries in enhancement of current-induced dynamics in magnetic bilayers. [Preview Abstract] |
Session D9: Elasticity and Geometry of Thin Objects I
Sponsoring Units: GSNPChair: Dominic Vella, DAMTP, Cambridge and LPS ENS, Paris
Room: 303
Monday, March 16, 2009 2:30PM - 2:42PM |
D9.00001: Can one hear a Kolmogorov Spectrum? Sergio Rica I will talk about a work in collaboration with G. During and C. Josserand on the long- time evolution of waves of a thin elastic plate in the limit of small deformation so that modes of oscillations interact weakly. According to the theory of weak turbulence (successfully applied in the past to plasma, optics, and hydrodynamic waves), this nonlinear wave system evolves at long times with a slow transfer of energy from one mode to another. We derived a kinetic equation for the spectral transfer in terms of the second order moment. We show that such a theory describes the approach to an equilibrium wave spectrum and represents also an energy cascade, often called the Kolmogorov-Zakharov spectrum. We perform numerical simulations that confirm this scenario. Finally, I will discuss recent experiments by A. Boudaoud and collaborators and N. Mordant. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D9.00002: Dynamical Origami Christophe Josserand, Arnaud Antkowiak, Basile Audoly, S\'ebastien Neukirch A drop falling on a thin elastic sheet is rapidly trapped after impact by self-folding of the sheet around the drop. This trapping process, due to capillary forces, occurs on the fast timescale of hydrophobic rebound. The resulting packed drop presents a complex three-dimensional shape, characteristic of the interplay between elasticity and capillarity (Py \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{98}, 2007). We study experimentally the encapsulation dynamics with high-speed video camera. A shape selection exhibited by the system is evidenced. The role played by the different parameters of impact (drop radius, impact velocity...) in the final shape of this ``dynamical origami'' is eventually discussed. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D9.00003: Mass distribution and geometry of a crumpled ball Anne Dominique Cambou, Narayanan Menon We use X-ray CT scanning to resolve in 3-dimensions the conformation of aluminum sheets with diameters L=7cm to 10cm and thickness T=25 microns, crumpled into spheres with diameters D=1.2cm to 1.5cm. The linear resolution of the reconstructed images is less than 6 microns/voxel. Measurements were made on spheres with average volume fractions, $\phi $ ranging from 0.06 to 0.11. The mass is not homogeneously distributed in the volume: when averaged over several samples, the volume fraction $\phi $(L/D) is found to increase with radius so that the sphere is densest at its surface. The radial dependence of volume fraction appears to be independent of average volume fraction and diameter, D. We also report preliminary measurements of the distribution of curvature in the sphere. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D9.00004: Stress relaxation in thin crumpled sheets Ingo Dierking, Paul Archer Compression of thin crumpled sheets subjected to a constant weight shows a wide range of scaling, covering up to five orders of magnitude [1], i.e. time scales from seconds to weeks. We demonstrate that this scaling behaviour is not smooth, but rather interrupted by sudden changes in height of the uniformly compressed crumple, which we attribute to sudden ridge collapses. Interestingly, when plotting the time laps between successive discontinuous ridge collapses as a function of time, the data falls onto a single linear functionality for all polymer film thicknesses, with a slope of d$\Delta $t/dt=1 over a scaling regime of four orders of magnitude.[2] Further, we investigate the scaling behaviour of thin sheets of different metals to elucidate a possible relation between the scaling parameter and the Young's modulus. Preliminary experiments suggest that scaling is a linear function of the elastic modulus. [1] K. Matan, R.B. Williams, T.A. Witten, S.R. Nagel, Phys. Rev. Lett., \textbf{88}, (2002), 076101. [2] I. Dierking, P. Archer, Phys. Rev. E, \textbf{77}, (2008), 051608. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D9.00005: The compensation of Gaussian curvature in developable cones is local Jin Wang, Thomas Witten We use the angular deficit scheme[1] to determine numerically the distribution of Gaussian curvature in developable cones(d-cones)[2] formed by forcing a flat elastic sheet into a circular container so that the sheet buckles. This provides a new way to confirm the vanishing of mean-curvature[3] at the rim where the sheet touches the container. This angular deficit scheme also allows us to explore the potential role of the Gauss-Bonnet theorem in explaining the mean-curvature vanishing phenomenon. The theorem's global constraint on curvature resembles the global conditions observed to be relevant for vanishing mean curvature. However, our result suggests that the Gauss-Bonnet theorem does not explain the vanishing of mean-curvature. \newline [1] V. Borrelli, F. Cazals, and J.-M. Morvan, {\sl Computer Aided Geometric Design} {\bf 20}, 319 (2003). \newline [2] E. Cerda, S. Chaieb, F. Melo, and L. Mahadevan, {\sl Nature} {\bf 401}, 46 (1999). \newline [3] T. Liang and T. A. Witten, {\sl Phys. Rev. E} {\bf 73}, 046604 (2006). [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D9.00006: Buckling Thin Disks and Ribbons with Non-Euclidean Metrics Christian Santangelo I consider the problem of a thin membrane on which a metric has been prescribed, for example by lithographically controlling the local swelling properties of a polymer thin film. While any amount of swelling can be accommodated locally, geometry prohibits the existence of a global strain-free configuration. To study this geometrical frustration, I introduce a perturbative approach. I compute the optimal shape of an annular, thin ribbon as a function of its width. The topological constraint of closing the ribbon determines a relationship between the mean curvature and number of wrinkles that prevents a complete relaxation of the compression strain induced by swelling and buckles the ribbon out of the plane. These results are then applied to thin, buckled disks, where the expansion works surprisingly well. I identify a critical radius above which the disk in-plane strain cannot be relaxed completely. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D9.00007: Fragmentation of an elastica Nicolas Vandenberghe, Emmanuel Villermaux When a thin rod is submitted to an axial force greater than its critical buckling load it takes the shape of an {\it elastica}. As the load further increases, a rod made of a brittle material breaks suddenly. More than two fragments may be formed during this fragmentation. In this work we discuss the sequence of events that lead to the final broken state with two or more fragments. We show that the criterion for breaking is not trivial. In particular, we investigate the effect of the duration of the loading and we show that at a given load the waiting time before breaking is broadly distributed. We discuss the consequences of the time delayed breaking on the distributions of fragment sizes and fragment numbers. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D9.00008: Relaxation of a plastic fold Morgan Cervo, Narayanan Menon Crumpled objects have been observed to show stress relaxation when confined to a constant volume, and to show creep when subjected to a constant load. These relaxation processes are described by logarithmic (or other similarly slow) functional dependences on waiting time. In an effort to understand the microscopic elements responsible for this slow collective relaxation, we study the mechanics of a single fold in a thin strip of polycarbonate sheet (typical dimensions: thickness t=0.127 mm, length L=14 cm, and width w=2cm). We create folds of different initial opening angles by placing the strip under varying loads. We then measure the opening angle as a function of time. We find that even one isolated fold is sufficient to mimic the relaxation behavior of the composite crumpled sheet: the unfolding process is logarithmic in time. The unfolding rate depends on sheet thickness, but surprisingly is independent of initial opening angle. We have observed qualitatively similar behavior in metal and paper sheets. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D9.00009: Shape and trajectory of a tumbling elastic sheet of paper Mike Robitaille, Arshad Kudrolli Inspired by wind dispersal of winged seeds and gliders, we study the flight of a tumbling piece of paper to explore the competing effect of inertia, lift, drag, and elasticity on its aerodynamics. Above a critical aspect ratio, a rigid rectangular sheet is well known to exhibit autorotation, leading to a lift force which causes it to drift away from the vertical as it falls through air. Less known is that the fact that the sheet buckles and bends along the axis of rotation when the rigidity of the sheet is reduced. We measure the deflection of the paper as a function of aspect ratio, and find its speed and angle of descent with high speed imaging. We find that the rotation speed is lower when the sheet is bent, than when it is unbent. The sheet deflection increases above a critical aspect ratio reaching a maximum before decreasing. The angle of descent is well described by a simple model balancing the gravitational, lift and drag forces acting on the sheet. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D9.00010: Shape and trajectory of a tumbling elastic sheet of paper II Daniel Tam, Michael Robitaille, Arshad Kudrolli, John Bush We investigate the dynamical coupling between the tumbling motion and elastic deformation of a paper strip freely falling in air. Recent experiments suggest the existence of a critical length above which the strip bends as it tumbles. We demonstrate that this bending is caused by the centripetal force associated with its tumbling motion. A simple theory predicts that bending occurs above a critical length in much the same way that buckling occurs in a compressed beam. We further discuss the influence of bending on the trajectory of paper strips, as well as biological implications for the dispersal of seed pods. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D9.00011: Nonlinear response of tensed membranes Peker Milas, Benny Davidovitch We study the response of elastic membranes under tension$ T$, to localized normal forces $F$. Focusing on simple geometries, characterized by translational or radial symmetries, we calculate the membrane shape for a range of values of F and T by numerically solving the appropriate FvK equations. We find that the linear regime, where membrane displacement is proportional to F, vanishes in the asymptotic limit$F/T<<1$, and characterize scaling properties of the resulting nonlinear response. We discuss the relevance of our results to the puzzling scaling behavior of the length of radial wrinkles, recently found in ``drop on membrane'' experiments (Huang\textit{ et al. }Science 2007). [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D9.00012: Period Fissioning and Other instabilities of stressed elastic membranes Benny Davidovitch We study the shapes of elastic membranes under the simultaneous exertion of tensile and compressive forces when the translational symmetry along the tension direction is broken. We predict a multitude of novel morphological phases in various regimes of a 2-dimensional parameter space$(\varepsilon ,\nu )$, defined by the relevant mechanical and geometrical conditions. The parameters $\varepsilon ,\nu $are, respectively, the ratio between compression and tension, and the wavelength contrast along the tension direction. In particular, our theory associates the repetitive period fissioining pattern, recently observed on wrinkled membranes floating on liquid and subject to capillary forces (J. Huang \textit{et al.}) to the morphology in the asymptotic regime $(\varepsilon <<1,\nu >>1)$where tension is dominant and the wavelength contrast is large. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D9.00013: Experimental study of the dynamics of crumpling Hillel Aharoni, Eran Sharon We experimentally measure the temporal evolution of crumpled configurations of thin elastic sheets. In our experiment, elastic hydrogel sheets swell inside a hard spherical shell, free of gravitational and plastic effects. We observe the dynamic evolution of structures in the sheet as confinement ratio increases, and analyze the statistical nature of the elastic energy localization around singularities. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D9.00014: Geometry Induced Charge Separation on a Helicoidal Ribbon Avadh Saxena, Victor Atanasov, Rossen Dandoloff Helical ribbons are ubiquitous in nature including in the carbon based nanostructures such as graphene. We derive an effective geometry-induced quantum potential for a particle confined on a helicoidal ribbon. This potential leads to the appearance of localized states at the rim of the helicoid. In this geometry the twist of the ribbon plays the role of an effective transverse electric field on the surface and thus this is reminiscent of the quantum Hall effect. We also calculate the effective polarization and discuss the consequences of these findings. [Preview Abstract] |
Session D10: Focus Session: Ferroelectrics III and Piezoelectrics
Sponsoring Units: DMPChair: Alexei Grigoriev, University of Tulsa
Room: 304
Monday, March 16, 2009 2:30PM - 2:42PM |
D10.00001: Light scattering answers precisely to the soft-mode in PbTiO$_{3}$ paraelectric phase Hwee Ping Soon, Hiroki Taniguchi, Yasuhiro Fujii, Mitsuru Itoh, Makoto Tachibana PbTiO$_{3}$ (PT) acts solely as the long-standing textbook example for the displacive-type phase transition; however, the precise observation of the soft-mode behavior of PT paraelectric phase still remains unavailable although it has been addressed by many research works [1-4]. In this study, we revisit the soft-mode behavior of PT single crystal by the confocal micro-Raman measurements. Opposing to the conventional belief that there occurs no first-order Raman scattering in the centrosymmetric PT paraelectric phase, the temperature dependence of the soft-mode has been precisely resolved for the first time due to the existence of macroscopic size Raman active regions ($\ge $ 780~nm). By evidently ruling out the possibility of defect-induced Raman scattering, the elasto-optical coupling serves as the most likely mechanism for the occurrence of these Raman active regions. [This work was supported by both KAKENHI (Grant No. 20248098) and Global COE program.] [1] G. Shirane, J. D. Axe, and J. Harada, Phys. Rev. B 2, 155 (1970). [2] M. Kempa et al., Phase Trans. 79, 351 (2006). [3] N. E. Tornberg and C. H. Perry, J. Chem. Phys. 53, 2946 (1970). [4] M. D. Fontana, H. Idrissi, and K. Wojcik, Europhys. Lett. 11, 419 (1990). [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D10.00002: Pretransitional Diffuse Neutron Scattering in Ferroelectric KTa$_{1-x}$Nb$_{x}$O$_{3}$ Grace Yong, Ross Erwin, Oleksiy Svitelskiy, Jean Toulouse, Lynn Boatner, Bernard Hennion, Stephen Shapiro Pretransitional diffuse neutron scattering in ferroelectric KTa$_{1-x}$Nb$_{x}$O$_{3}$ occurs at (110), (111), and (130) with no diffuse scattering at (100), and (200). These observations will be discussed using the elastic structure factor and `uniform phase shift' description. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D10.00003: Nonlinear Driven Response of a Phase-Field Crystal in a Periodic Pinning Potential Cristian Achim, Jorge Ramos, Mikko Karttunen, Ken Elder, Enzo Granato, Tapio Ala-Nissila, See-Chen Ying We study numerically the phase diagram and the response under a driving force of the phase field crystal model for pinned lattice systems in one and two dimensions. The model describes the lattice system as a continuous density field in the presence of a periodic pinning potential, allowing for both elastic and plastic deformations of the lattice. We first present results for phase diagrams of the model in the absence of a driving force. The nonlinear response to a driving force on an initially pinned commensurate phase is then studied via overdamped dynamic equations of motion for different values of mismatch and pinning strengths. For large pinning strength the driven depinning transitions are continuous, and the sliding velocity varies with the force from the threshold with power- law exponents in agreement with analytical predictions. Transverse depinning transitions in the moving state are also found in two dimensions. Surprisingly, for sufficiently weak pinning potential we find a discontinuous depinning transition with hysteresis even in one dimension under overdamped dynamics. We also characterize structural changes of the system in some detail close to the depinning transition. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D10.00004: Mesoscopic nanostructures and long-time relaxation processes in ferroelectrics with coexisting ferroelectric and antiferroelectric phases V. Sobolev, V. Ishchuk, Z. Samoilenko Presentation contains results of experimental study of the kinetics of mesoscopic segregate structures formed in the vicinity of interphase domain boundaries separating the domains of the coexisting ferroelectric and antiferroelectric phases in two series of the lead zirconate titanate based solid solutions in which the isovalent complex (La$_{0.5 }$Li$_{0.5})^{2+}$ or the La$^{3+}$ ions are substituted for lead. These systems of solid solutions possess an extended interval of compositions for which the inhomogeneous state of coexisting ferroelectric and antiferroelectric domains is energetically favorable in comparison with homogeneous ferroelectric and antiferroelectric phases. The mechanisms that control the kinetics of formation of the segregates are investigated and compared for these two series of solid solutions [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D10.00005: Accurate calculations of the full NMR chemical shielding tensor in perovskites: B3LYP embedded cluster results for $^{17}$O and $^{47, 49}$Ti Daniel L. Pechkis, Eric J. Walter, Henry Krakauer B3LYP calculations of $^{17}$O and $^{47, 49}$Ti NMR in BaTiO$_3$ and SrTiO$_3$ will be presented. These systems were modeled with finite size quantum clusters embedded in classical fields generated from arrays of point charges and pseudopotentials. For polar clusters, an external E-field was applied to eliminate spurious depolarization fields. The full $^{17}$O chemical shielding tensors, $\hat{\sigma}$(O), in BaTiO$_3$ and SrTiO$_3$ are in very good agreement with recent precise single crystal measurements.\footnote{ R. Blinc, et al., J. Phys: Cond. Matt. {\bf 20}, 085204 (2008).} The calculated Ti isotropic chemical shielding values are within the range of all reported measurements of BaTiO$_3$. Results will also be presented for the solid solution PZT series. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D10.00006: Local ordering and structural instabilities in (Na,Bi)TiO$_3$ perovskites Sahak Petrosyan, Marco Fornari, Boris Kozinsky, Nicola Marzari, Gerbrand Ceder Density functional theory is used to predict structural and electronic properties of lead-free piezoelectric titanate (Na,Bi)TiO$_3$. We studied the energetics of different phases in the phase diagram and investigated the tendency to local ordering on the A-site. In order to elucidate the mechanism for the phase transitions and the piezoelectric performance we also probed the dependence of ferrodistortive and antiferrodistortive instabilities upon pressure and chemical substitutions. Our results point to local tetragonal ordering and provide insight on possible strategies to improve the electromechanical properties. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D10.00007: Density-functional study of the Ba$_{x}$Bi$_{(1-x)}$(M$_{(1-x)/2})$Ti$_{(1+x)/2}$O$_3$ perovskite solid solution Dennis Jackson, David Roundy Using density-functional theory we predict properties of the solid solution perovskites Ba$_{x}$Bi$_{(1-x)}$(M$_{(1-x)/2})$Ti$_{(1+x)/2}$O$_3$ where M is Mg, Ni or Zn. These properties are strongly affected by the presence of both A-site and B-site disorder. We study all distinct orderings within a given $2\times2\times2$ supercell. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D10.00008: Predicting new structures in B-cation ordering perovskites Matthew Lords Material properties are intimately tied to crystal structure. Many materials, alloys in particular, share a common, underlying motif, such as an fcc, bcc, or hcp parent lattice but have different chemical orderings on the lattice. For example, the well-known structure in the Cu-Au system, L1{\_}0, has an underlying fcc lattice where each (001) plane contain only Cu or Au. The planes are stacked so that the Cu and Au layers alternate. Among the countably infinite possibilities for such chemical orderings, why does nature choose only the few it does? Are others possible or likely? We answer this question generally and give an example of the perovskite structure, important in ferroelectrics, catalysts, and superconductors. We show which structures are possible combinatorially and which might actually be observed in real materials. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D10.00009: First principles study of piezorotation in thin-film LaAlO$_3$ Alison Hatt, Nicola Spaldin We investigate the effect of biaxial strain on [001] thin-film LaAlO$_3$ using density functional calculations. We find that the essentially rigid rotations of the AlO$_6$ octahedra vary linearly with strain, a phenomenon which may be described as piezorotation by analogy with piezomagnetism or piezoelectricity [S. Denev, et al., Phys. Rev. Lett. 100, 257601 (2008).] Within a small range of experimentally achievable strain values, we find that epitaxial strain can stabilize several distinct modes of cooperative rotations not found in the bulk, and identify first order phase transitions between states with different piezorotative responses. Finally, we investigate how the electronic and elastic properties are affected by proximity to the strain-induced phase transitions. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D10.00010: Nano-electromechanical systems based on a piezoelectric single crystalline thin film on Silicon Jonghoo Park, Dustin J. Kreft, Robert H. Blick, Seung-Hyub Baek, Chang-Beom Eom, V. Vaithyanathan, Darrell G. Schlom, Vladimir Aksyuk Nano-electromechanical systems (NEMS) have shown great progress and promise as sensors and actuators. In spite of great progress, efficiency and integration techniques for actuating and tuning NEMS has remained a challenge. We have employed a single crystalline piezoelectric thin film on a silicon substrate to obtain a high piezoelectric coefficient and high electromechanical coupling for a NEMS device. The suspended NEMS device consists of Pt/Pb(Mg$_{x}$Nb$_{1-x})$-PbTiO$_{3}$/SrRuO$_{3}$/SiTiO$_{3}$ and is clamped at the ends via silicon substrate. Pt and SrRuO$_{3}$ are used as top and bottom electrodes, respectively, and SiTiO3 serves as a buffer layer to the silicon substrate. We have shown experimentally that the piezoelectric actuation based on PMN-PT devices consume less power and are more responsive than other NEMS devices of similar nature and size. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D10.00011: Structural Study of PMN-xPT by Neutron Diffraction D. Phelan, P.M. Gehring, Q. Huang, Z.-G. Ye, C. Stock, G. Xu, J. Wen Stark differences between x-ray and neutron measurements of the structures of ferroelectric-relaxors PMN-xPT ((1-x)Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$+xPbTiO$_3$) and PZN-xPT (Z=Zn) have been reported [1]. One explanation for these differences is that these crystals have strained surface layers on the order of several tens of $\mu$m thick, the crystal structure of which differs from that of the crystal bulk. This phenomenon has been coined the ``anomalous skin effect'' but has been recently challenged [2] and thus remains controversial. We reinvestigated the skin effect in PMN-xPT by considering the possibility that the oxygen stoichiometry might play a role. Two sets of powders (x=0.1, 0.2, 0.3, and 0.4) were grown, one with and one without oxygen annealing, and high resolution neutron powder diffraction measurements were carried out for both sets. For a given x, both sets of powders have the same structural phase, suggesting that the effects of oxygen annealing are minimal. For x=0.1 and x=0.2 both sets of powders are rhombohedral, which contrasts with the single crystal neutron diffraction measurements. This supports a skin effect in that the grain size of the powders is small enough that the Bragg peaks are dominated by the strained surface layer. References [1] G. Xu et al., Phase Transitions 79, 135 (2006) [2] E. H. Kisi and J.S. Forrester, J. Phys.:Condens. Matter 17, L381 (2005) [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D10.00012: Complex local structures in lead based perovskite relaxors Guangyong Xu, Zhijun Xu, Jinsheng Wen, Peter Gehring, Chris Stock The lead-based, perovskite relaxors Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$ (PZN), Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$ (PMN) and their solid solutions with PbTiO$_3$ (PT) continue to receive significant attention because they exhibit huge piezoelectric responses and are therefore technologically important. It is widely accepted that many of the special properties of relaxors are related to local (charge, chemical, and polar) order. In particular, short-range polar order, a.k.a. ``polar nano-regions'' (PNR), is believed to appear in relaxors at temperatures well above the Curie temperature T$_C$. The PNR contribute to the frequency dispersion of the dielectric properties and have recently been suggested to be associated with the high piezoelectric response. We have performed series of neutron diffuse scattering measurements on PMN-x\%PT and PZNx\%PT single crystals. Our results indicate that the local polar structure is complicated, having T1 and T2-type components with different polarizations. In particular, the T1 component with $\langle$001$\rangle$ polarization can be partially suppressed by an [001] field, the T2 component with $\langle$110$\rangle$ polarization can be affected by a [111] field. The T2 component also exhibits a strong coupling to the acoustic phonon while the T1 component is associated with the polar optic phonon. We will discuss the complex nature of the local structure in relaxors and their implications. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D10.00013: First-Principles Study of Diffuse Scattering in Pb(Sc$_{1/2}$Nb$_{1/2})$O$_{3}$ P. Ganesh, M. Ahart, R.E. Cohen, E. Cockayne, B. Burton Recent X-ray and neutron experiments show diffuse scattering with characteristic butterfly and rod shapes in relaxor ferroelectrics Pb(Sc$_{1/2}$Nb$_{1/2})$O$_{3 }$(PSN) and Pb(Zn$_{1/2}$Nb$_{1/2})$O$_{3 }$(PZN) [1]. We have simulated diffuse scattering in PSN using first-principles based simulations in the ferroelectric, relaxor and the paraelectric phases [2]. The model assumes quenched chemical order in the form of chemically ordered regions in a chemically disordered matrix. Below the Burns temperature, polar nanoregions (PNR) form, pinned spatially to the COR, but whose polarization evolve dynamically. In the relaxor phase we find ``butterfly'' shaped diffuse scattering around the [100] peaks and around the [110] peaks we find ``rod'' shaped diffuse scattering similar to experiments. As the system is driven towards the paraelectric phase, the diffuse patterns around all the Bragg peaks display radial streaks elongated towards the origin. We have determined that the rods and butterflies below the Burns temperature originate from the PNR, while the radial streaks above it arise from atomic displacements associated with chemical disorder. References: [1] G. Xu, Z. Zhong, H. Hiraka and G. Shirane, Phys. Rev. B, \textbf{70}, 174109 (2004) [2] S. Tinte, B. P. Burton, E. Cockayne and U. V. Waghmare, Phys. Rev. Lett., \textbf{97}, 137601 (2006) [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D10.00014: High-pressure x-ray diffraction and Raman scattering studies of Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$-xPbTiO$_{3 }$ Mukhtar Ahart, Maddury Somayazulu, Ronald Cohen, Russell Hemley Motivated to determine and understand the compositional-pressure phase diagram for PMN-xPT solid solutions, we employed the angular dispersive x-ray diffraction methods (Advanced Photon Source, Argonne National Laboratory) and high pressure Raman scattering to investigate a series of PMN-xPT solid solutions (x=0.2, 0.3, 0.33, 0.35, 0.37, 0.4) in a diamond anvil cell up to 20 GPa. The x-ray diffraction results show changes in Bragg peaks at about 6 or 7 GPa which indicate that PMN-xPT systems undergo a ferroelectric to a paraelectric phase transition. In addition, a new peak centered at 380 cm$^{-1}$ appears above 6 GPa for all the samples. The morphotropic phase boundary (x=0.33 to 0.37) with the monoclinic symmetry persists up to 7 GPa. Based on our results, we suggest a possible structure for high pressure phase, which is R3c. We suggest a phase diagram for PMN-PT system which is slightly different from the one predicted by B. Chaabane et al (Phys. Rev. B 70, 134114, 2004). [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D10.00015: Molecular dynamics computational studies of relaxor ferroelectric behavior in Pb(Mg$_{1/4}$Ti$_{1/4}$Nb$_{1/2}$)O$_{3}$ (PMN-PT) Hiroyuki Takenaka, Ilya Grinberg, Andrew Rappe Relaxor ferroelectrics are of fundamental scientific interest and are also used in a variety of applications, such as piezoelectric transducers and capacitors. They exhibit permittivity peaks that are broad with respect to both temperature and frequency. We have developed a bond-valence model for Pb(Mg$_{1/4}$Ti$_{1/4}$Nb$_{1/2}$)O$_{3}$ (PMN-PT) and performed atomistic bond-valence molecular dynamics (BVMD) simulations of PMN-PT. We have studied relaxor behavior at a range of temperatures, in order to analyze polar nanoregion dynamics and relaxation lifetimes. We find that even for a fairly small simulation size of $6\times6\times6$ supercell (1080 atoms), the system exhibits frequency dispersion. We present the results of $6\times 6 \times 6$, $8\times 8\times 8$ and $10\times 10\times 10$ supercell BVMD simulations, analyze the pair distribution function of the PMN-PT and elucidate the local chemical origin of relaxor behavior. [Preview Abstract] |
Session D11: Optical Properties of Dots, Wires and Superlattices
Sponsoring Units: DCMPChair: Michael Scheibner, Naval Research Laboratory
Room: 305
Monday, March 16, 2009 2:30PM - 2:42PM |
D11.00001: Robust Ferromagnetism in type-II (ZnMn)Te Quantum Dots I.R. Sellers, V.R. Whiteside, M. Eginligil, R. Oszwaldowski, I. Zutic, A. Petrou, B.D. McCombe, W.C. Chou Temperature dependant magneto-photoluminescence studies of type-II diluted magnetic semiconductor (ZnMn)Te/ZnSe quantum dots (QDs) will be presented. As expected, the exchange interaction between the Mn spins and charge carriers results in a strong optical polarization of the luminescence at low temperature in a magnetic field. In addition, however, a \underline {zero magnetic field} optical polarization degree of 7{\%} is observed in the photoluminescence (PL). This polarization is shown to be independent of temperature until 180 K, and is only quenched by the loss of PL intensity as the type-II QDs are ionized. In this submission, we will present continuous wave and temporal PL measurements which indicate that the finite polarization, at zero magnetic field results from the formation of exciton magneto polarons (EMPs). Furthermore, we will show that these EMPs are remarkably robust with binding energies in excess of 40 meV, far larger than any previously studied EMP system. The origin of this behavior will be discussed. Work supported in part by CSEQuIN and the Office of the Provost at the U. Buffalo. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D11.00002: Low threshold amplified spontaneous emission and Auger recombination suppression in giant nanocrystal quantum dots Istvan Robel, Florencio Garcia-Santamaria, Richard D. Schaller, Yongfen Chen, Jennifer A. Hollingsworth, Victor I. Klimov Nanocrystals quantum dots (NQDs) are attractive materials for various light-emitting applications including optical amplification and lasing. A complication associated with the multiexcitonic nature of light amplification is NQDs is the picosecond optical-gain decay induced by nonradiative Auger recombination, in which one exciton recombines by transferring the energy to the other. Here, we present new results on a novel type of nanocrystals dubbed ``giant'' quantum dots (g-NQDs). These g-NQDs comprise an emitting core particle of CdSe overcoated with a thick shell (up to 20 monolayers) of wider-gap CdS. We report that biexciton and gain lifetimes are greatly augmented and the ASE threshold drops down to just a few $\mu $J/cm2. We explain this result by a significant increase in the absorption cross-section of g-NQDs compared to traditional nanocrystals and lengthening of biexciton lifetimes. We also observe other unusual optical-gain behaviors for these structures such as multi-band ASE, the spectra spectral range of optical amplification extends over more than 500 meV. These results demonstrate that g-NQDs are very promising materials for applications in practical lasing technologies. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D11.00003: Investigation of group-III-nitride semiconductor nanostructures using an eight-band $\mathbf{k}\cdot\mathbf{p}$ formalism Oliver Marquardt, Tilmann Hickel, Joerg Neugebauer Despite its approximative nature, the $\mathbf{k}\cdot\mathbf{p}$-formalism provides a numerically efficient and accurate description of the electronic structure of group-III nitride semiconductor nanostructures with characteristic dimensions of few nanometers. [1] With the computational effort of this method being independent of the system size, it is possible to study an extensive set of zero-, one- and two-dimensional semiconductor nanostructures. We applied a plane-wave implementation of the 8-band $\mathbf{k}\cdot\mathbf{p}$ formalism and second-order continuum elasticity theory to various III-nitride nanostructures such as InGaN/GaN or GaN/AlN quantum dots in the characteristic wurtzite and zincblende crystal structures. We investigated the effect of strain and polarization effects on the charge carrier localization which typically leads to a spatial separation of electrons and holes in wurtzite nanostructures. Additionally, studies have been performed in order to evaluate trends when varying the alloy composition in InGaN/GaN nanostructures in order to understand light emission processes in realistic nanostructures.\\ \\ 1: Marquardt, Mourad, Schulz, Hickel, Czycholl, Neugebauer, Phys. Rev. B, in print (2008) [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D11.00004: Infrared Photodetectors based on PbSe and PbS Nanoparticles. Don-Hyung Ha, Christopher Murray There is growing interest in developing nanoparticles for photodetectors, due to the potential to selectively tune the wavelengths detected by varying the size of the nanoparticles. Also, by virtue of the solution processibility of nanoparticles, photodetectors based on nanoparticles provide a low cost, easily processed opportunity for photodetection on flexible substrates. Especially for the near-infrared (NIR) region, PbS and PbSe nanoparticles are ideal candidates since their absorption windows fall between 900-1500nm and 1400-2500nm, respectively, covering a wide range of the IR region. This presentation reports the synthesis, structural characterization, and photoconductivity of colloidal PbSe and PbS nanoparticles. Photocurrent and normalized detectivity are measured from the nanoparticle photodiode under the illumination of NIR light (800-2500 nm) and under dark conditions. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D11.00005: Single photon to single electron conversion using a quantum dot Haruki Kiyama, Takafumi Fujita, Tetsuya Asayama, Akira Oiwa, Seigo Tarucha Photons, and electron spins are leading candidates for implementing qubits useful in information transmission, and computing, respectively. Therefore, quantum media conversion (QMD) between them is a key technology for a comprehensive quantum network. In this work, as a first step toward QMD, we demonstrate single photon to single electron (charge) conversion using a GaAs based lateral QD equipped with a quantum point contact (QPC) as a charge sensor. A distinctive step, which is quite similar to those observed for single electron tunneling onto the QD from the leads, is observed in the single-shot time evolution of the QPC current immediately after the incidence of single photons. From detailed measurements of the light intensity dependence and the QD-lead tunnel rate dependence we confirm that the observed steps are due to single electron generation in the QD just after single photon irradiation. The minimum time resolution of this single photon to single electron conversion is 50 $\mu $s. This is short enough to demonstrate the angular momentum transfer between photon polarization and electron spin in the QD as the next step. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D11.00006: Scalable Single Photon Detector for Terahertz and Infrared Applications. Bernard Matis, Dong Ho Wu Recent advancements in the research areas of quantum dot (QD) and single electron transistors (SET) open up an exciting opportunity for the development of nanostructure devices. Of the various devices, our attention is drawn in particular to detectors, which can respond to a single photon over a broad frequency spectrum, namely, microwave to infrared (IR) frequencies. Here we report on transport measurements of parallel quantum dots, fabricated on a GaAs/AlGaAs 2-dimensional electron gas (2DEG) substrate, under the influence of external fields associated with 110GHz and 1 THz signals. We further investigate the scalability of our detector in addition to its temperature dependence up to 4.2K. We will discuss experimental results, and their dependence on quantum dot size, and fabrication techniques, as well as the limitations in developing a QD photon detector for microwave and IR frequencies, whose noise equivalent power can be as high as 10$^{-22}$ W/Hz$^{1/2}$. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D11.00007: Measurement of the separation dependence of the resonant energy transfer between CdSe nanocrystallite quantum dots Farbod Shafiei, Ricardo S. Decca An apparatus has been built to study the separation dependence of the interaction between small and large resonant groups of CdSe/ZnS nanocrystallite quantum dots (NQDs). A near-field scanning optical microscope (NSOM) is used to bring a group of mono-disperse 6 nm dots close (near-field range) to an 8 nm group of dots which are deposited on a solid immersion lens. Combination of spectral and positional filtering allows us to measure the interaction between small numbers of NQDs, with the ultimate goal of identifying the interaction between individual dots. Quenching of the small NQDs photoluminescence signal yields the transition probability between these two groups of NQDs which is obtained to be $(4.5\times10^{-47}$ m$^6$)/R$^6$, matching the theoretical calculation. F\"{o}rster radius as a signature of energy transfer efficiency is extracted from experimental data to be 17 nm. Separation between two groups of the NQDs was increasing up to 40nm during the experiment. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D11.00008: Resonant creation of positive trion in coupled InAs/GaAs quantum dots Swati Ramanathan, Kushal C. Wijesundara, Mauricio Garrido, Eric A. Stinaff, A. S. Bracker, D. Gammon Recent photoluminescence excitation (PLE) experiments have revealed the unexpected resonant creation of a positive trion in a coupled InAs/GaAs quantum dot system. Positive trion creation is a two photon process requiring the second photon to have a different energy from the first due to the presence of the photogenerated hole. This leads us to conclude that the positive trion may be created through two indirect absorptions, along with two tunneling events. To verify this scenario, experiments using circularly polarized excitation should result in hole spin states with either spin -3/2 or +3/2. This should lead to Pauli blocking of spins, resulting in a reduced intensity of positive trion emission under excitation with circularly polarized light when compared to unpolarized light. High resolution PLE will also provide additional insight into the details of this mechanism. It may be possible to use resonant excitation processes such as this to create defined hole spin states in coupled quantum dot systems. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D11.00009: Exchange interactions in coupled quantum dots observed through polarized photoluminescence Kushal C. Wijesundara, Mauricio Garrido, Swati Ramanathan, Eric Stinaff, Allan Bracker, Dan Gammon Identification and manipulation of the exchange interaction between different spin configurations may be useful for implementing quantum logic operations. Coupled quantum dots offer the possibility of controlling the exchange interaction by continuously tuning between direct and indirect excitonic configurations. The effect of the anisotropic part of the exchange energy was clearly identified from polarization dependent photoluminescence (PL) results arising from the direct and indirect configurations of the neutral exciton. We also observe direct experimental evidence of the isotropic exchange interaction via PL measurements from positive trion configurations. The isotropic exchange interactions observed to be an order of magnitude larger than the anisotropic splitting as expected. High resolution measurements of this charged exciton configurations are expected to reveal additional insight into the details of the exchange interaction. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D11.00010: Hybrid Exciton in a Semiconductor Nanorod Coated By an Organic Shell Daniel Velazquez, Huong Nguyen We study the Wannier Mott-Frenkel hybrid exciton in a nanorod coated by a thin organic shell. Using the wavefunctions of the 1D Wannier-Mott and the Frenkel exciton, we obtain the wavefunctions and energy of the hybridization state. The new exciton state is a linear combination of the basic exciton states and is smoothly distributed over the whole system. The hybridization depends strongly on the coupling (hybridization) parameter as well as the shape of the nanorod and the thickness of the organic layer. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D11.00011: Stimulated Raman Scattering from Short GaP Nanowires Jian Wu, Awnish Gupta, Peter Eklund We report an interesting discovery of very strong non-linear optical behavior in short GaP nanowire segments. They were formed by cutting a $\sim $40 $\mu $m long and 210 nm diameter GaP nanowire into various lengths using a focused ion beam. This approach allows us to study length as the variable in the non-linear behavior. A giant nonlinear Raman amplification has been observed in these segments with length L$<$ 1.2$\mu $m for the first time. The nonlinear Raman effect has been demonstrated to increase as the lengths of nanowire segments decreases. As far as the relationship between Raman scattering intensity and laser pump power, we also observed that there exists a threshold pump laser power which separates the linear and super-linear regions. The effective pump power can be as low as 200 $\mu $W that is 1000 times smaller than bulk values. We attribute this giant nonlinear Raman effect to stimulated Raman scattering (SRS) from nanocavities formed by these short GaP nanowires. The quality factor Q of these short segments was estimated to be 10$^{3}$ to 10$^{4}$. We believe our observation suggests the possibility to make a new type of SRS semiconductor laser. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D11.00012: Polarized Rayleigh and Raman Study of single CuO Nanowire Qiujie Lu, Jian Wu, Humberto Gutierrez, Timothy Russin, Peter Eklund Crystalline CuO is an interesting Ferroelectric and Ferromagnetic system which we have recently grown in nanowire (NW) form. In this paper, we present results of Raman and Rayleigh scattering studies of individual CuO NWs to probe optical antenna effects that we first discovered in GaP NWs. We have shown that these antenna effects can, in general, strongly mask Raman selection rules in semiconducting nanowires[1]. Using a microRaman spectrometer, polarized light scattering experiments (backscattering geometry) were carried on NWs suspended over holes in a TEM grid. TEM was therefore also used to identify the growth axis and determine the NW diameter. As a function of the angle q between the NW axis and the incident laser field, we collect the Rayleigh scattering intensity as well as the Raman LO and TO optical phonon scattering intensity. These results can then be used to quantify the optical antenna effects in the CuO system. NWs of different diameters, from 70nm to 200 nm were studied; the results depend dramatically on the NW diameter. Our results will be compared to EM calculations based on the DDA approximation. This work is supported by NSF NIRT, grant DMR-0304178. [1] Chen G. Jian Wu, etc., Nano Lett. 2008 Vol.8 pp. 1341-1346. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D11.00013: Polarized Rayleigh and Raman Back-scattering from Individual GaP Nanowires Duming Zhang, Jian Wu, Qiujie Lu, Humberto Gutierrez, Peter Eklund Results of polarized Rayleigh and Raman back-scattering studies are reported for individual $\sim $20 $\mu $m long crystalline GaP Nanowires (NWs) using 514.5 nm excitation. The NWs were supported over holes in TEM grids. The diameters and growth directions of the NWs were thereby determined by TEM and the same NW could be studied optically. Peak positions of characteristic LO, TO phonon Raman bands were found to agree with bulk GaP. Both the Rayleigh and Raman back-scattering intensity polar patterns $I$(\textit{$\theta $}) were measured at room temperature, where \textit{$\theta $} is the angle between the incident electric field and the NW axis. The scattered radiation was polarized parallel to the incident electric field. From the Rayleigh back-scattering intensity polar patterns, the factor in the scattered radiation was obtained. Together with the Raman tensor determined from the growth direction of the NWs, the Raman back-scattering intensity polar pattern was calculated for each case and correlated with the experimental data. Our measurement on the Rayleigh and Raman back-scattering intensity polar patterns reveled different patterns ranging from dipole-like to symmetric (circular or elliptical) depending on the NWs growth directions and diameters. This work is supported by NSF- NIRT, grant DMR-0304178. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D11.00014: Optical properties of Nb:SrTiO$_{3}$/SrTiO$_{3}$ superlattices Woo Seok Choi, Soon Jae Moon, Hiromichi Ohta, Byung Cheol Jeon, Jong Hoon Shin, Yun Sang Lee, Tae Won Noh Recently, oxide heterostructures are being extensively investigated as an effort to understand unusual physics occurring at the oxide interfaces. For example, formation of 2DEG or strongly modified electronic structures at the interface between two different oxide constituents have been examined carefully and understood to some extent. In this contribution, we studied the electronic properties and phonon dynamics of Nb:SrTiO$_{3}$/SrTiO$_{3}$ superlattices. Using optical spectroscopy, we obtained the optical conductivity for a wide range of photon energy (3.7 meV -- 6 eV). We could separately identify the free carrier response and the phonon dynamics of the superlattice. The carrier density and unusually small scattering rate were obtained varying temperature as well as the superlattice period. In addition, we could reveal a strong electron-phonon coupling from the existence of the spectral weight in mid-IR photon range. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D11.00015: Effect of Magnetic Field on Broadening of Excitonic Spectra in Superlattices Yuri Khavin, Nikolai Sibeldin, Mikhail Skorikov, Vitaliy Tsvetkov, Daniel Oberli, Alok Rudra, Romain Carron, Eli Kapon We present a comprehensive study of optical properties of GaAs/AlGaAs superlattices (SLs) with different tunneling coupling between wells in magnetic fields in a wide range of excitation levels. Photoluminescence (PL) and PL excitation spectra demonstrate significant broadening of the exciton lines with increasing excitation power. Several features indicate that the exciton line widths are mainly determined by excitonic scattering. In particular, in zero magnetic field, the heavy hole (hh) PL line broadens symmetrically with increasing excitation power. In nonzero in-plane magnetic field, the exciton binding energy strongly increases (by a factor of 2 in 14 T), and the dependence of the line width on excitation power becomes much weaker. In strong in-plane fields, significant shift of the hh PL line towards lower energies is observed under high excitation levels. It is possible that this shift is a manifestation of interparticle interactions in an electron-hole system. [Preview Abstract] |
Session D12: Novel Instrumentation and Techniques in Surface Science
Sponsoring Units: DMP DCMPChair: MIchael Dreyer, University of Maryland
Room: 308
Monday, March 16, 2009 2:30PM - 2:42PM |
D12.00001: A Combined Scanning Tunneling Microscope-Quartz Crystal Microbalance Investigation of Heating and Liquid-Like Behavior at a Sliding Interface Benjamin Dawson, Jacqueline Krim The unique capabilities resulting from combining a scanning tunneling microscope and a quartz crystal microbalance have been used to characterize the heating and wear at the interface of a tungsten tip and an Indium substrate, with a change in the contact characteristics of the interface occurring for sufficient sliding speeds. The advantage of this system is the ability to probe subtle changes of a rubbing asperity contact, which will aid in developing a more complete understanding of the complex issue of heat generated via friction. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D12.00002: Dynamics and Spreading of pentanol and other alcohols for MEMS applications Brendan Miller, David Hook, Jacqueline Krim Microelectromechanical Systems (MEMS) have the potential to revolutionize widespread technologies, but tribological issues are currently preventing commercialization of some devices. Self-assembled monolayers (SAMs), while highly effective against release related stiction, are ineffective as MEMS lubricants [1]. Vapor phase lubrication has been proposed as a solution to the issue of tribological failure in MEMS with alcohol vapors attracting much interest. In an effort to understand the basic mechanisms of lubrication we have performed a quartz crystal microbalance (QCM) study of the uptake, sliding friction, and spreading rates of adsorbed alcohols on silicon and SAM treated substrates. [1] D. A. Hook, S. J. Timpe, M. T. Dugger, and J. Krim. \textit{Tribological degradation of fluorocarbon coated silicon microdevice surfaces in normal and sliding contact. }J. Applied Physics \textbf{104} (2008). [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D12.00003: Numerical Studies of Friction Between Metallic Surfaces and of its Dependence on Electric Currents Evangelos Meintanis, Michael Marder We will present molecular dynamics simulations that explore the frictional mechanisms between clean metallic surfaces. We employ the HOLA molecular dynamics code to run slider-on-block experiments. Both objects are allowed to evolve freely. We recover realistic coefficients of friction and verify the importance of cold-welding and plastic deformations in dry sliding friction. We also find that plastic deformations can significantly affect both objects, despite a difference in hardness. Metallic contacts have significant technological applications in the transmission of electric currents. To explore the effects of the latter to sliding, we had to integrate an electrodynamics solver into the molecular dynamics code. The disparate time scales involved posed a challenge, but we have developed an efficient scheme for such an integration. A limited electrodynamic solver has been implemented and we are currently exploring the effects of currents in the friction and wear of metallic contacts. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D12.00004: Combined conducting atomic force/scanning Kelvin probe microscope for investigating charge trapping on semiconductor surfaces James Moore, Sean Kenny, Monika Ruchala, Mikhail Reshchikov, Alison Baski A novel combination of conducting atomic force microscopy (CAFM) and scanning Kelvin probe microscopy (SKPM) was used to investigate charge trapping and transfer on semiconductor surfaces. CAFM is first used to locally inject charge at an oxide/semiconductor interface, and then SKPM is performed to monitor the evolution of the resulting surface potential. In a dark environment, the additionally charged interface states due to CAFM charge injection result in additional band bending that persists for hours in GaN, ZnO and Si. Specifically for GaN, a model based on a tunneling mechanism was used to describe the CAFM charge injection, where electrons travel from the tip through an oxide barrier and become trapped at the interface. The decrease in induced band bending with time shows a logarithmic behavior, similar to transients produced after exposure to light. This combination of techniques offers a relatively simple method for investigating induced band bending on semiconductor surfaces and could become a useful tool for understanding concentrations of charged surface states. Specifically, current collapse in GaN FETs and HFETs has been linked to concentrations of charged surface states at the contacts. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D12.00005: Hot Electron Transport Properties of Thin Copper Films Using Ballistic Electron Emission Microscopy J.J. Garramone, J.R. Abel, I.L. Sitnitsky, L. Zhao, I. Appelbaum, V.P. LaBella Copper is widely used material for electrical interconnects within integrated circuits and recently as a base layer for hot electron spin injection and readout into silicon. Integral to both their applications is the knowledge of the electron scattering length. To the best of our knowledge, little work exists that directly measures the scattering length of electrons in copper. In this study we used ballistic electron emission microscopy (BEEM) to measure the hot electron attenuation length of copper thin films deposited on Si(001). BEEM is a three terminal scanning tunneling microcopy (STM) based technique that can measure transport and Schottky heights of metal/semiconductor systems. We find a Schottky height of 0.67~eV and an attenuation length approaching 40~nm just above the Schottky height at 77~K. We also measure a decrease in the attenuation length with increasing tip bias to determine the relative roles of inelastic and elastic scattering. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D12.00006: A Comparative Study of Au-Au, Ru-Ru and RuO$_{2}$-Au RF MEMS Contacts in Controlled Vacuum Environments Matthew Walker, J. Krim, N. McGruer We have constructed an UHV system with in situ oxygen plasma cleaning capabilities in order to observe the impact of film contamination in reproducible conditions. We have performed measurements to allow comparison of soft, hard and combined soft-hard contacts. All switches are closed before applying a potential across the contacts to minimize e-field evaporation and material transfer between contacts. Prior to, and for short O2 plasma exposure times, the initial contact resistance measurements had larger variations. With longer O2 plasma exposure times initial and extrapolation resistances measurements converged. These results are consistent with prior reports, which showed that the oxide layer on a Ru surface thickens with exposure to O2 plasma [1]. Therefore, under stringent experimental conditions, we have demonstrated that operating RF MEMS contacts are comparable to those studied in the surface science literature. [1] Y. Iwasaki, A. Izumi, H. Tsurumaki, A. Namki, H. Oizumi, I. Nishiyama, \textit{Appl. Surf. Sci. }\textbf{\textit{253}}$, 2007$ [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D12.00007: Reduction of Barrier Height at Close Proximity between two Gold (111) surfaces. Yoshifumi Oshima, Yoshihiko Kurui Work function is one of important physical properties in order to understand electron emission or chemical reactions such as catalysis. Work function changes locally depending on a defect, step or adsorbate on the surface. A scanning tunneling microscope (STM) is one of powerful tools to investigate such a local work function, which is called a local barrier height. In this method, the barrier height is defined as $\phi =\left( {1/1.025\ast d\ln G/dz} \right)^2$, where G is conductance and z, the tip-surface distance. Experimentally, the barrier height has been reported to be constant independent of the distance. But, theoretically, it is suggested to be reduced at close proximity. In this study, we investigated the distance between two gold (111) surfaces in TEM observation simultaneously with measuring conductance value in a process of approaching each other. The distance changes controllably by a piezo actuator when it is above 1 nm, but the distance becomes narrower that the expected value obtained by a piezo actuator when it is below 1 nm. Obviously, structural relaxation is occurred when the distance between two gold surfaces is below 1 nm. Taking the structural relaxation into an account, we confirm that the barrier height is reduced at close proximity of two gold (111) surfaces. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D12.00008: Coherent X-ray Surface Diffraction: Speckle from the Surface Reconstruction Layer of Gold (001) Michael S. Pierce, Kee-Chul Chang, Daniel Hennessy, Alec Sandy, Michael Sprung, Hoydoo You We present preliminary results of the first coherent x-ray diffraction from an atomic monolayer surface reconstruction and demonstrate how this technique is capable of providing new information about surface dynamics. Speckled scattering patterns were successfully collected from ordered surface atoms on Au (001) crystals in high vacuum. We have collected data at the (001) anti-Bragg point as well as coherent scattering data directly from the in-plane hex reconstruction peak. These two points can provide complementary information about in-plane and out-of-plane surface dynamics. Below 1050 K, the system appears to remain in equilibrium maintaining a small constant fraction of non-reconstructed surface for a given temperature. However we observe the speckles continue to evolve within the collected scattering patterns indicating that the non-reconstructed portion of the surface rearranges slowly. Above about 1050K, we find that the rate of speckle decorrelation rapidly increases for very small changes in temperature. Signal to noise makes this a difficult experiment with existing light sources. However we expect this technique to become easier and more broadly applicable with future X-ray sources. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D12.00009: In-situ chemical and structural characterization via RHEED-total reflection analysis of x-rays (TRAXS). Sandeep Chandril, Cameron Keenan, Thomas Myers, David Lederman The use of x-ray fluorescence produced inside a molecular beam epitaxy chamber by the RHEED electron gun to simultaneously characterize the thin films for thickness, roughness and the chemical composition is described. This technique requires only slight modifications to the chamber and can be a powerful tool for beam flux calibration and in-situ analysis, especially where surfaces have to be protected under vacuum and for the stoichiometry control during growth. The angular dependence of the x-ray fluorescence signal from the thin film over the substrate is analyzed using Parratt's approach and simulating electrons' trajectories inside the film to account for grazing angle electron beam as a source for x-rays. We have found good agreement between the experiment and the theory for the thickness and roughness estimates. Experiments for chemical composition determination are currently underway. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D12.00010: High Energy XRD/XRF for High-Throughput Phase Mapping of Composition Spread Thin Films John Gregoire, Darren Dale, Alexander Kazimirov, Michele Tague, Hector Abruna, Francis DiSalvo, R. Bruce van Dover Analysis of thin film inorganic libraries is an increasingly popular technique for materials discovery and optimization. For ternary and higher-order libraries, the high-throughput determination of the crystalline phase fields is an active field of research due to its importance in understanding a given material system. We discuss our techniques for high-throughput data acquisition and analysis using a 60keV x-ray source at the Cornell High Energy Synchrotron Source. The techniques provide simultaneous mapping of the composition, crystalline phase, and fiber texture of a composition spread thin film. We also demonstrate the utility of this data in interpreting our measurements of the thin film's catalytic activity for the oxidation of methanol. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D12.00011: Low Energy Electron Microscope Imaging of Doped Si Structures Buried under Thermal Oxides Gary Kellogg, Meredith Anderson, Craig Nakakura We present recent progress towards low energy electron microscope (LEEM) imaging of doped-silicon, diode test structures buried under thermally grown oxides. The question addressed here is whether the observed contrast at incident electron energies just above the vacuum cutoff is due to differences in doping type or oxide thicknesses. To circumvent complications arising from charging of the when exposed to the imaging electron beam, we developed a method to measure ``pre-charging'' current voltage (IV) curves and applied it to three test samples with oxide thicknesses varying from 2.8 to 50 nm. The vacuum cutoff energies obtained from the IV curves depend on both doping type \textit{and} oxide thickness and are strongly influenced by external factors including surface contamination and UV exposure. The time dependence of the oxide charging increases significantly with oxide thickness providing further insights into the origins of LEEM contrast. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D12.00012: LEED structural analysis of strongly correlated systems: reaching the limit of the instrumentation? TeYu Chien, Biao Hu, Shuheng Pan, V. Braun Nascimento, E. Ward Plummer In strongly correlated systems, like transition-metal oxides and the Iron Pnictides, subtle changes in structural parameters can cause dramatic changes in the physical properties. Breaking the symmetry by creating a surface is a controlled way to explore the structure-functionality relationship. Low Energy Electron Diffraction (LEED) $I-V $has been one of the most used surface structural techniques, but there are inherent and instrumental limitations which will be discussed using data from surfaces of transition-metal compounds. Using CCD cameras and new data analysis three dimensional plots of diffraction vs parallel momentum can be created. Spot intensity, width, position, and profile, as well as the diffuse background can then be quantitatively extracted and evaluated. We will show how to couple the STM with LEED using data from the parent superconducting compound BaFe$_{2}$As$_{2}$. The inherent limitations of the existing system are tested using Cu(100). [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D12.00013: Electron emission from surfaces resulting from low energy positron bombardment S. Mukherjee, K. Shastry, A. H. Weiss Measurements of the energy distribution of electrons resulting from very low energy positron bombardment of a polycrystalline Au and Cu(100) surfaces provide evidence for a single step transition from an unbound scattering state to an image potential bound state. The primary positron energy threshold for secondary electron emission and cutoff in the secondary electron energy spectra are consistent with a process in which an incident positrons make a transition from a scattering state to a surface-image potential bound while transferring all of the energy difference to an outgoing secondary electron. Estimates of the probability of this process as a function of incident positron energy are also presented. Background free Auger spectra of the MVV transition in Cu and the OVV transition in Au were obtained by setting the incident positron beam energy below the secondary electron emission threshold. Auger electron emission resulted from the annihilation of surface state positrons with core electrons. The low energy tail associated with the low energy CVV Auger transitions in Cu and Au were found to have integrated intensity several times larger than Auger peak providing strong evidence for multi-electron Auger processes. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D12.00014: Hydrogen absorption by a Pd film detected by microgravimetry J.I. Avila, R. Trabol, U.G. Volkmann, A.L. Cabrera, C. Romero, P. Lievens A thin film of palladium (200 {\AA}) was e-beam evaporated onto a quartz crystal used in a SQM-160 Microbalance from Sigma Instruments. The crystal was then successively exposed to different hydrogen pressures, in situ, and the change in the resonant frequency was recorded as a function of time. Hydrogen absorption by the Pd film can be obtained by the shift of the resonant frequency upon hydrogenation at different pressures. A 14 Hz shift is obtained when the Pd film is saturated with hydrogen at 10 Torr of pressure. The different faces of the Pd-H system are observed with this technique. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D12.00015: Observation of In Plane Magnetization Reversal Using Polarization Dependent Magnetooptical Kerr Effect Hendrik Ohldag, Franz Hillebrecht We present an experimental setup for in plane two axis magnetometry by employing the polarization dependence of the magnetooptical kerreffect. (MOKE). The proposed approach allows for observing the complete in plane reversal process during a hysteresis loop. For this purpose a conventional setup to measure longitudinal MOKE with crossed polarizers is extended by a Faraday cell to compensate for the rotation of the polarization vector caused by the magnetized sample. This detection scheme enables us to observe hysteresis loops of single monolayer. Using a Jonesmatrix formalism we are able derive expressions for the kerr rotation using oblique incident polarization, allowing for extracting 2-dimensional vectorial information about the magnetization reversal process in the plane of the sample surface. The approach can be further extended to extract all three components of the magnetization by acquiring more than two loops. Since this setup does not require to change the sample geometry {\em in situ} it can be easily attached to an existing ultra high vacuum setup. [Preview Abstract] |
Session D13: SPS Undergraduate Research II
Chair: Gary White, American Institute of PhysicsRoom: 309
Monday, March 16, 2009 2:30PM - 2:42PM |
D13.00001: SESSION BREAK |
Monday, March 16, 2009 2:42PM - 2:54PM |
D13.00002: Phase measurements on a subwavelength optical metamaterial based on metallic paired strips Kara Maller, Thomas Jarvis, Xiaoqin Li, Dmitriy Korobkin, Gennady Shvets, Marcelo Davanco, Xuhuai Zhang, Stephen R. Forrest There is no known naturally occurring material with negative index of refraction because the electrical and magnetic resonances in naturally occurring materials do not overlap in frequency. However, artificially engineered materials, known as metamaterials, can be designed to exhibit such peculiar properties. We study a subwavelength optical metamaterial composed of paired gold strips separated by a continuous gold film. According to theoretical calculations, this structure is expected to display a negative index of refraction in the near-infrared. We perform phase measurements of the material using a polarization interferometer and a tunable femtosecond laser. The phase information of the transmitted wave at various wavelengths is used to characterize the index of refraction. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D13.00003: Undergraduate Research with Josephson Qubits: From Fabrication to Spectroscopy Alyssa Wilson, Jerome Mlack, Anthony Tyler, Zechariah Thrailkill, Joseph Lambert, Roberto Ramos Josephson junctions are scalable solid state devices that can be used as qubits in quantum computing. In this talk we will describe the different stages involved in the fabrication, characterization and state measurement of a particular superconducting qubit known as the Josephson phase qubit. We have performed current-voltage measurements that determined the critical current of our device and produced histograms of switching currents that were needed to establish the quantum state of the junction. We will also report on the progress on microwave spectroscopy measurements involving multiple qubits coupled using on-chip capacitors. Spectroscopy reveals the various energy levels of entangled quantum states. We will discuss how physics undergraduates have contributed to this work. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D13.00004: Degradation Studies of Polymer Blend Photovoltaics Brian Johnson, Enaanake Allagoa, Robert Thomas, Greg Stettler, Marianne Wallis, Justin Peel, Brian McNelis, Richard Barber We have measured the time dependence of device performance for photocells using blends containing the conjugated polymer, Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with different functionalized C$_{60}$ electron acceptor molecules: commercially available [6,6]-Phenyl C$_{61}$ butyric acid methyl ester (PCBM) or [6,6]-Phenyl C$_{61}$ butyric acid octadecyl ester (PCBO) produced in our laboratory. Performance was characterized by the maximum power output of the devices, with the time dependence typically following an exponential decay. Variations in the characteristic lifetime of the devices were observed to depend on the molar fraction of the electron acceptor molecules (calculated with respect to the MEH-PPV monomer fraction). Differences in the decay behavior between the PCBM and PCBO blends will be presented. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D13.00005: Coulomb Blockade I-V Characteristics in Nanowires Sarah Joy, Guneeta Singh-Bhalla, Arthur Hebard, Amlan Biswas, Selman Hershfield The current-voltage (I-V) characteristics of manganite nanowires seen in an experiment closely resemble the hallmark I-V curves of the Coulomb blockade. Unlike normal Coulomb blockade curves, these curves have multiple lines instead of one, branching out from the zero current point. In order to discover an explanation for these I-V curves a simulation based on the Coulomb blockade was done for multiple junctions, because the experiments were done in a regime where multiple islands of ferromagnetic material are separated by insulating regions, as a result of intrinsic phase separation. The simulation results show I-V curves that are in good qualitative agreement with the experiment. The branching of I-V curves is due to a large change in resistance between the islands -- not a change in the capacitance. Analytic work shows that the change in the junction resistance is too large to be explained by the tunneling magnetoresistance, but consistent with a change in the barrier thickness or composition. Simulations of the effect of gates show that the low voltage regime changes periodically with the gate voltage, while the high voltage regime is independent of gate voltage. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D13.00006: A quartz tuning fork as a force sensor for atomic force microscopy Arthur Ianuzzi, Julia Neff, John Timmerwilke, Amlan Biswas We are designing and building an atomic force microscope (AFM) to characterize the surface properties of perovskite oxides in low temperature ($\sim $10K) and high magnetic field (9T) environments. We are using a quartz tuning fork as the force sensor. The z-axis displacement of a conducting AFM tip due to surface features will be detected by observing the shift in resonance frequency of the tuning fork which is attached to the tip. The conducting tip will also allow us to perform conductive atomic force microscopy. The resonance characteristics of the tuning fork element were determined in various degrees of vacuum, with and without the tip installed, and as a function of the tip's proximity to the sample surface. We show that the high resonance frequency ($\sim $32kHz ) and quality factor ($\sim $30,000) of the tuning fork makes it an extremely sensitive force detector. The apparatus has also been designed with the capability of performing magnetic force microscopy on perovskite oxides. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D13.00007: Mossbauer Study of the xCr2O3-(1-x)$\alpha $-Fe2O3 nanoparticles system. Sean Krupa, Monica Sorescu The xCr2O3-(1-x)$\alpha $-Fe2O3 nanoparticles system was synthesized hydrothermally for x=0.0 to x=0.9. M\"{o}ssbauer spectroscopy was performed on the obtained samples as well as for samples subjected to thermal annealing at 550 C for one hour. At x=0.1, the as obtained samples began showing superparamagnetism and became completely superparamagnetic after x=0.4 concentration. The percent of the sample that was superparamagnetic increased with Cr2O3 substitution. This correlates with chromium decreasing the particle size of the powder. The thermally annealed samples appeared to have the hematite structure re-grown for concentrations x=0.1 to x=0.4 with the intensity of the hyperfine magnetic field decreasing with Cr2O3 concentration. For x=0.5 to x=0.9, the percent of superparamagnetic particles increased with Cr2O3 concentration, dominating the system by x=0.8. This system is believed to have applications in gas sensing and catalysis. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D13.00008: Construction of a semiconductor laser system for magneto-optical trapping of atomic rubidium Ian Dougherty, Michael Lim We report on the construction of a semiconductor laser system to generate light for magneto-optical trapping of rubidium. A DFB laser is used as a master oscillator with rapid frequency tuning accomplished by an acousto-optic modulator in double-pass configuration. The resulting beam seeds a 0.5-W tapered semiconductor amplifier chip. We spatially mode-match its output to a polarization-maintaining, single-mode fiber using lenses and two anamorphic prism pairs. At maximum efficiency the system generates more than 100 mW of TEM00 power at the fiber output. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D13.00009: Combinatorial Synthesis of (V$_{1-x}$,Cr$_{x})_{2}$GeC Epitaxial Films C. E. Steinmetz, J. R. Applegate, S. M. Benjamin, B. D. Adamson, J. D. Hettinger, S. E. Lofland, T. H. Scabarozi We report the synthesis of solid solutions of (V$_{1-x}$,Cr$_{x})_{2}$GeC epitaxial films on single crystal Al$_{2}$O$_{3}$ epi-polished substrates. The materials are sputtered from four cathodes; three are powered by rf-power supplies while one is driven by a dc-power supply. The materials are co-deposited at a temperature of 900\r{ }C and we get solubility across the whole range of compositions. The substrate-film interaction creates a strain which makes the new phase energetically favorable. The surface structure was measured using atomic force microscopy revealing that the composition has little impact on this property. We found a new MAX-phase material, (V$_{0.5}$Cr$_{0.5})_{4}$GeC$_{3}$ , which has never been synthesized in bulk form. We report the conditions required to make these materials. In addition, electrical transport characteristics as a function of composition will be reported. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D13.00010: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:30PM - 4:42PM |
D13.00011: Thermodynamic Properties of Ising Spins on the Triangular Kagom\'{e} Lattice Bilin Zhuang, Courtney Lannert The triangular Kagom\'{e} lattice can be constructed by inserting a lattice site on each bond of the Kagom\'{e} lattice. Each unit cell contains 6 $a$-sites, 3 $b-$sites, 6 \textit{aa-}bonds and 12 \textit{ab-}bonds. The lattice with antiferromagnetic \textit{aa}-bonds is known to exhibit geometrical frustration at low temperatures. We applied analytical methods and Monte Carlo simulations to study a system of Ising Spins on the lattice and investigated its thermodynamics properties. In particular, the heat capacity of the model exhibits interesting features based on the strength and the sign of coupling constants J$_{aa}$ and J$_{ab}$. In the case when the \textit{aa-}interaction is antiferromagnetic, the heat capacity shows two broad peaks at kT/$\vert $J$_{ab}\vert \cong $1.8 and at kT/$\vert $J$_{aa}\vert \cong $1.8. In the case when the \textit{aa-}interaction is ferromagnetic, the heat capacity shows a sharp peak at kT/$\vert $J$_{ab}\vert \cong $1.9 and another low broad peak at around kT/$\vert $J$_{aa}\vert \cong $1.4. We also studied a much simpler system of two $a-$trimers connected with $b-$sites to reproduce the thermodynamics behaviors of the more complicated triangular Kagom\'{e} lattice and to further understand the origin of its interesting properties. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D13.00012: Monte Carlo Simulations of a $111$-Ising Dipolar Model on the Hyper-kagom\'{e} Lattice Patrick Carter, John Hopkinson, Matthew Enjalran Motivated by the physics of dipolar spin ice, we investigate the hyper-kagom\'{e} lattice with $111$-Ising spins. The hyper-kagom\'{e} lattice can be generated by a selective removal of one site per tetrahedron of the pyrochlore lattice to yield a lattice of corner-sharing triangles. The spin-$1/2$ Ir$^{4+}$ ions of Na$_4$I$_3$O$_8$ represent an experimental realization of the hyper-kagom\'{e} structure. We report preliminary results from Monte Carlo simulations of the $111$-Ising dipolar hyper-kagom\'{e} model, which represents a yet to be studied disorder-free limiting case of diluted spin ice. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D13.00013: Thermal measurements of the MAX-phase material Cr$_{2}$GeC D. T. Piwowar, P. Abbazia, D. Filomena, D. Dorofy, M. Garzon, S. E. Lofland, J. D. Hettinger, T. H. Scabarozi We have measured the specific heat and thermal transport in bulk Cr$_{2}$GeC samples that have been hot-isostatically-pressed or hot pressed. We observe no difference in the results based upon the synthesis procedure. We find the low temperature specific heat is fit well by the standard expression. These measurements suggest a large density of states at the Fermi level or a large electron-phonon coupling in this material. The Debye temperature, extracted from the phonon contribution to the specific heat, is found to be 460 K which roughly correlates with the Debye temperature extracted from the ultrasonic elastic measurements. The phonon contribution to the thermal conductivity will also be reported. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D13.00014: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 5:18PM - 5:30PM |
D13.00015: Borosilicate films as permeability barriers J. R. Applegate, C. E. Steinmetz, J. D. Hettinger, J. F. Carroll, R. Krchnavek Borosilicate films have been deposited using rf-sputtering techniques from a composite target at room temperature onto polypropylene (PP), high density polyethylene(HDPE), low density polyethylene(LDPE), and polyethylene terephthalate (PETG) substrates. Films were found to be smooth, flexible, with excellent adhesion to the substrates. Repeated rolling the coated substrates on a radius of 0.5mm resulted in no discernable damage for films less than 200nm in thickness. Creasing the substrates did result in local damage. However excellent adhesion did not allow the fractured glass to come off the substrate. Heat generated during deposition only influenced the films grown on LDPE where the thermal expansion mismatch between the film and substrate induced strains caused fractures in thick films. Modifications to processing parameters allowed thick films to be grown without fractures. Permeability measurements of nitrogen resulted in significant improvements in comparison to uncoated substrates. [Preview Abstract] |
Session D14: Colloids II: Structure and Rheology
Sponsoring Units: DFDChair: David Pine, New York University
Room: 315
Monday, March 16, 2009 2:30PM - 2:42PM |
D14.00001: Lock and Key Colloids Stefano Sacanna, William Irvine, Paul Chaikin, David Pine We demonstrate a recognition mechanism between microscopic (colloidal) particles based on a simple ``lock-and-key" principle that relies only on the complementary morphology of the particles involved. The system we developed consists of charge-stabilized spherical silica colloids (keys) and specially designed polymeric particles with spherical cavities (locks). The assembly of locks with keys is driven by depletion interactions between the particles and an uncharged water soluble polymer (poly-ethylene oxide). We show that by balancing electrostatic repulsion and depletion attraction, we induce a selective and reversible lock-and-key self-assembly. Moreover, we can design the lock and key single units to have separate functionalizable chemistries, such that the resulting composite particle (lock+key) will exhibit anisotropic surface properties. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D14.00002: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 2:54PM - 3:06PM |
D14.00003: Pairwise Additivity in Colloidal Electrostatics Jason W. Merrill, Sunil K. Sainis, Eric R. Dufresne We present a method for measuring electrostatic and hydrodynamic interactions between colloidal particles based on observations of short-time trajectories. We use this method to explore whether forces between colloidal particles can be considered pairwise additive by comparing the force between a pair of particles to forces between groups of several particles. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D14.00004: Structure and dynamics of suspensions of nanoparticles in nematic liquid crystals Brian Gettelfinger, Gary Koenig, Jose Moreno-Razo, Juan Hernadez-Ortiz, Nicholas Abbott, Juan de Pablo A hierarchical modeling approach has been adopted to examine the structure and dynamics of nanoparticles suspended in liquid crystals. A molecular model is used to predict the defects that arise in nanoparticle assemblies, as well as their response to applied flow fields. The model is solved by resorting to a radial basis function based technique. The validity of the model and our numerical results are established by direct comparison to results from molecular dynamics simulations of nanoparticles in nematic and isotropic liquid crystals. Results for particle diffusion and aggregation at equilibrium and in flowing systems are then used to interpret our experimental data for a variety of systems. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D14.00005: Monodisperse polymethyl methacrylate (PMMA) spheres in organic media: synthesis update Andrew Hollingsworth, Mark Elsesser, William Irvine, David Pine, Paul Chaikin Since the publication of Antl, et al. [Colloids and Surfaces 17 (1986) 67--78] more than 20 years ago, several research groups in the soft condensed matter area have attempted the dispersion polymerization of sterically-stabilized PMMA particles. Most have found that success of this particular synthesis depends critically on the quality of the comb-graft stabilizer, poly(12-hydroxystearic acid)-g-PMMA. More recent work has extended the particle synthesis to include the incorporation of covalently attached, fluorescent dyes in the particle interior. Our goal has been to reproduce some of these results-- a challenging task-- and to improve the process, leading to a reliable method for preparing core-dyed PMMA particles. We will report on several important findings related to this research goal, and demonstrate that our particles can be used to make colloidal clusters via the recently published emulsion encapsulation and shrinkage technique [Science 301, 483--487 (2003)]. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D14.00006: A model nanocolloidal rod system to explore structural transitions in networks and bundles Georgina Wilkins, Patrick Spicer, Michael Solomon We introduce a model system consisting of self-assembled polyamide anisotropic colloids suspended in aqueous surfactant solutions. The colloidal particles are formed by precipitation from an amorphous polyamide powder that is dispersed with mechanical agitation in an aqueous surfactant phase at temperatures from 59 to 100$^\circ$C. The aspect ratio increases monotonically with temperature: at $\textrm{T} = 59^\circ \textrm{C}$, short rods with aspect ratio r $= 8 \pm1$ form. At $\textrm{T} = 100^\circ \textrm{C}$, rope like structures with r $= 306 \pm14$ form. By confocal laser scanning microscopy (CLSM) and dynamic light scattering (DLS) as volume fraction is increased we show a structural transition from dilute rod behaviour with diffusive dynamics to a homogeneous network structure with increasingly slow dynamics. Furthermore, increasing the aspect ratio of rods induces the same structural transition from dilute rod behaviour to a network structure. Finally, we vary the interaction potential between the rods by a polymer induced depletion interaction and observe an unexpected quiescent network to bundle transition. The bundles are several rod diameters wide and 1 - 2 rod lengths long. The rods appear to be ordered nematically within each bundle. The bundling transition leads to an order of magnitude decrease in the storage modulus of the suspensions. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D14.00007: Polymer grafted particles: Architectural effects on the dynamics Panayiotis Voudouris, Jihoon Choi, Hong Dong, George Fytas, Michael Bockstaller, Kris Matyjaszewski We present a combined static and dynamic light scattering study of two polystyrene PS{\&}SiO$_{2}$ particle solution systems in which tuning of the grafting density and molecular weight of the surface bound PS afford intermediate (0.5nm$^{-2})$ and concentrated (0.84nm$^{-2})$ brush densities. The different packing environment of PS chains give rise to distinctively different rich dynamic response above a threshold volume fraction that yields insight into the role of polymer grafts on the structure formation of hairy particles. This work is the first report on the missing dynamics of hybrid core-shell nanoparticles with distinct behavior intermediate between ultra soft multiarm star polymers and hard sphere colloids. With increasing grafting density of PS ligands the dynamic properties approach those of hard sphere systems while retaining some of the polymer-specific dynamic characteristics. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D14.00008: Structure of Quasi-One Dimensional Ribbon Colloid Suspensions Binhua Lin, Stuart A. Rice, Thomas Stratton, Bianxiao Cui We report the results of an experimental study of a colloid fluid confined to a quasi-one dimensional (q1D) ribbon channel. Our findings confirm the principal predictions of previous theoretical studies of such systems. These are (1) that the density distribution of the liquid transverse to the ribbon channel exhibits stratification and (2) that even at the highest density the order along the strata, as measured by the longitudinal pair correlation function, is characteristic of a liquid. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D14.00009: Dynamics of Internal Stresses and Scaling of Strain Recovery in Aging Colloidal Gels Ajay Singh Negi, Chinedum Osuji On cessation of flow, dilute suspensions of carbon black particles undergo rapid gelation and display instantaneous residual or internal stresses which relax slowly with time. We monitor the evolution of these stresses (under zero strain) and find a weak power law decay, $\sigma _{i}\sim $t$_{w}^{\alpha }$ over 5 decades of time where $\alpha \approx $0.1. The system exhibits aging, with the elastic modulus scaling as a weak power law of elapsed time, G'$\sim $t$^{\beta }$, with $\beta \approx \alpha $. Imposition of zero stress conditions after waiting time t$_{w}$, at internal stress $\sigma _{i}$(t$_{w})$, results in strain recovery as the system relaxes without the zero strain constraint. Older systems exhibit less recovery than younger ones. Remarkably, strain recoveries at different t$_{w}$ can be shifted to construct a single master curve in which the magnitude of the recovery is shifted vertically according to $\sigma _{i}$(t$_{w})^{-1}$ and horizontally simply with elapsed time. The scaling of the strain recovery with internal stress suggests that the internal stress state is characteristic of the age of the system and of the manner in which the system will continue to evolve. This result has important implications for our understanding of glassy behavior in soft materials. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D14.00010: Synthesis and Self-assembly of Janus and Patchy Particles by Lift-up Microcontact Printing Shan Jiang, Steve Granick Janus and patchy particles were synthesized by a simple and novel lift-up microcontact printing method. The geometry of the particles is revealed by both optical fluorescence microscopy and scanning electron microscopy. It is demonstrated that the Janus balance (geometry) of the particles can be easily fine tuned. Interesting and unique cluster structures were self-assembled from particles synthesized by this method. The method allows particles not only of divalent but also of trivalent geometry to be formed in large quantity. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D14.00011: Thermal Properties of Particulate Suspensions Rebecca Christianson, Jessica Townsend It has been known since the 1800's that addition of solid phase particles to a liquid can improve the thermal conductivity of the liquid. However, the instability of such suspensions made them impractical for cooling applications. With the advent of affordable technology for synthesizing nanometer scale particles, it became possible for stable suspensions with improved thermal properties to be created. Initial investigations of nanoparticle suspension coolants (termed nanofluids) seemed to indicate an anomalous enhancement of the thermal conductivity above that predicted by conventional theories. However, subsequent experimental work showed issues with the reproducibility of these early results, which has been attributed by some sources to aggregation within the suspensions. I will present work from our group studying the properties and practical application of stable nanoparticle suspensions, as well as our initial findings on the effects of aggregation on the measured thermal properties of particulate suspensions. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D14.00012: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:54PM - 5:06PM |
D14.00013: Adiabatic Intramolecular Movements for Water Systems in Ab initio simulations Luana Pedroza, Antonio J.R. da Silva The detailed description of hydrogen bonds in water is essential to understand the great variety of processes that occur in this system. Clearly the most appropriate way to do that description would be to treat all the degrees of freedom quantum mechanically. Another possibility is to treat the nuclei classically and the electrons quantum mechanically. A very commom approximation in these simulations is to consider each water molecule as a rigid body, which clearly imposes a limitation on the real description of the molecules and their interactions. We here present an effective treatment of the intramolecular degrees of freedom of water, where these modes are decoupled from the intermolecular one, adiabatically allowing these coordinates to be positioned at their local minimum of the PES. This decoupling is performed combining an AIMC simulation using the rigid bodies approximation with an intramolecular optimization. As an application of our methodology we have studied small water clusters. We show that even in the case of the water dimer the sampling of phase space is significantly modified when intramolecular optimization is included (J. Chem. Phys., 128, 104311 (2008)). As a result, there are clear changes in features such as the dipole moment and structural properties. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D14.00014: New look of fractional exclusion statistics Dragos-Victor Anghel I discuss the concept of fractional exclusion statistics and I show that it leads to inconsistencies in the calculation of the particle distribution that maximizes the partition function. These inconsistencies appear when mutual exclusion statistics is manifested between different subspecies of particles in the system. In order to eliminate these inconsistencies, I introduce new mutual exclusion statistics parameters, which are proportional to the dimension of the Hilbert sub-spaces on which they act. These new definitions lead to properly defined particle distributions and thermodynamic properties. I also show that fractional exclusion statistics is manifested in general interacting systems and I calculate the exclusion statistics parameters. Most importantly, I prove that indeed, the mutual exclusion statistics parameters are proportional to the dimension of the Hilbert space on which they act. \\[4pt] {\bf Related publications:}\\[0pt] [1] D. V. Anghel, J. Phys. A: Math. Theor. {\bf 40}, F1013 (2007).\\[0pt] [2] D. V. Anghel, Phys. Lett. A {\bf 372}, 5745 (2008).\\[0pt] [3] D. V. Anghel, arXiv:0804.1474. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D14.00015: Capillary forces on nanowires Jun Ma, Shengfeng Cheng, Jay Wallace, Patricia McGuiggan, Mark Robbins The capillary forces on nanowires have been measured by attaching them to the cantilever of an Atomic Force Microscope (AFM). The nanowires are immersed and retracted from a liquid/air interface. The entire capillary force curve is compared to continuum theory and molecular simulations. Nanowires with different diameters and chemistry and various liquids are investigated. Surface tension, contact angle hysteresis, and dynamical contact angles can be extracted under reasonable assumptions about how the contact line moves along the nanowires. [Preview Abstract] |
Session D15: Monolayers, Membranes and Microemulsions
Sponsoring Units: DFDChair: Erik Luijten, Northwestern University
Room: 316
Monday, March 16, 2009 2:30PM - 2:42PM |
D15.00001: Nanoparticle--induced domain formation in zwitterionic lipid membranes Meenakshi Dutt, Erik Luijten Charged nanoparticles have been found to induce coexistence of gel and fluid states in a pure zwitter\-ionic membrane\footnote{Wang et al., Proc.\ Natl.\ Acad.\ Sci.\ (in press).} due to electrostatic interactions between the nanoparticle and the adjacent membrane monolayer. Analogies can be drawn between this system and biological membranes, where domains in the vicinity of charged peripheral proteins play an important role in regulating cell activity.\footnote{Bergelson et al., Mol.\ Memb.\ Biol.\ \textbf{12}, 1 (1995).} To understand the differences in the structural and dynamical properties of coexisting domains in lipid membranes, we develop a coarse-grained model that represents the monolayer as a quasi two-dimensional plane of dipoles. The electrostatic interactions between the charged nanoparticle and the adjacent membrane monolayer, as well as the steric interactions within the monolayer, are incorporated explicitly. We will discuss our model and present results on the thermodynamic and structural changes in the model membrane induced by the presence of the nanoparticle. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D15.00002: ABSTRACT HAS BEEN MOVED TO Y36.00013 |
Monday, March 16, 2009 2:54PM - 3:06PM |
D15.00003: Langmuir films of chiral lipid molecules and Pattern Formation . Prem Basnet, Elizabeth Mann, Sahraoui Chaieb Langmuir films of 1,2-bis(10,12 Tricosadiynoyl)-sn-Glycero-3-Phosphoethanolamine form spiral and target patterns when compressed between two movable barriers in a Langmuir trough above 30$^{0}$C, up to the chain-melting transition at $\sim $37$^{0}$C. The critical pressure, at which spirals appear, increases with temperature. The patterns themselves also depend on temperature, with single-armed spirals with many defects forming near 30$^{0}$C and defect-free target patterns at higher temperatures. The mechanism of spiral formation could be a competition among elasticity, chirality, and the boundary conditions at the core of the domains. Optical anisotropy and the growth rate of internal structures test this suggested mechanism. . [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D15.00004: Phase behavior of a binary phospholipid/cholesterol Langmuir monolayer: comparison of Brewster angle and fluorescence microscopy Fanindra Bhatta, Pritam Mandal, David Allender, Elizabeth Mann, Yasmin Isler, Edgar Kooijman, Andrew Bernoff The binary mixture of dihydrocholesterol and dimyristoyl phosphatidylcholine exhibits two liquid phases at the air/water interface: a cholesterol-rich and a phospholipid-rich phase, with a well-known critical point at a critical composition. Approaching that point with increasing monolayer pressure, the differences between phases disappear, along with the line tension between phases. In our experiments, the line tension is determined through comparison of the relaxation of domain shapes towards equilibrium with a compact, numerically tractable boundary integral model for the system hydrodynamics. We use both fluorescence microscopy and Brewster angle microscopy to visualize the lipid monolayer, and find the two methods have significant differences in monolayer behavior near the critical point. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D15.00005: Large-scale free-standing monolayer membranes of nanoparticles: preparation and properties Jinbo He, Laszlo Frazer, Xiao-Min Lin, Adam Weis, Heinrich Jaeger Two-dimensional arrays of close-packed nanoparticles can be stretched across tens-micrometre-size holes. The resulting freestanding monolayer membranes extend over hundreds of particle diameters without crosslinking of the ligands or further embedding in polymer. In our previous results of dodecanethiol-ligated 6-nm-diameter gold nanocrystal monolayers, we find a Young's modulus of the order of several GPa. This remarkable strength is coupled with high flexibility, enabling the membranes to bend easily while draping over edges. Recently we found that oleic-acid-covered cobalt nanoparticcles ($\sim $9 nm in diameter) self-assemble at toluene/ethylene glycol interfaces and form large two-dimensional arrays. These membranes stretch across tens-of-micrometer holes after drying of ethylene glycol. The mechanical and diffusion properties of these membranes are tested and the response of these membranes under external fields is also investigated. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D15.00006: Casimir force between inclusions in a strechable fluid membrane Hsiang-Ku Lin, Roya Zandi, Leonid Pryadko We calculate the entropic fluctuational force, a finite- temperature analogue of the Casimir force, between foreign inclusions in a strechable fluid membrane. Specifically, we consider the fluctuations of a planar membrane governed by the full Helfrich Hamiltonian, including the surface tension and both bending rigidity terms. The inclusions are introduced as circular regions where the surface tension and/or bending rigidities are modified from their values on the non-perturbed membrane. Results for arbitrarily-strong perturbations of the membrane, including holes, rigid disks, and edges will be presented. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D15.00007: Breathable NIPAAm Network with Controllable Hydration Supports Model Lipid Membrane Michael Jablin, Hillary Smith, Mikhail Zhernenkov, Ajay Vidyasagar, Ryan Toomey, Jessica Saiz, Boris Toperverg, Erik Watkins, Tonya Kuhl, Alan Hurd, Jaroslaw Majewski The interaction of a model lipid bilayer composed of DPPC with a surface-tethered poly(N-isopropylacrylamide) (NIPAAm) was explored with neutron reflectometry (NR). The Langmuir-Blodgett / Langmuir-Schaeffer method was used to deposit a lipid bilayer onto the polymer. NR measurements were used to probe the in- and out-of-plane structure of the system as a function of temperature. NR with fluorescence microscopy show that the polymer supports a lipid bilayer, and hydration of the support can be controlled. At low temp. the membrane develops out-of-plane undulations visible in off-specular scattering. Analysis of the off-specular reveals in-plane correlation of the bilayer fluctuations. The separation of the lipid bilayer from the solid support of a substrate constitutes a significant step towards a more realistic model of biological membranes. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D15.00008: The effects of lung surfactant peptide mimic KL4 on lipid monolayer collapse Niels Holten-Andersen, Luka Pocivavsek, Alan J. Waring, Ka Yee C. Lee We have investigated the origin of the positive effect of the peptide KL4 on lung surfactant lipid monolayers containing DPPC and POPG. Using surface balance techniques and fluorescence microscopy we have observed that KL4 rigidifies POPG containing lipid monolayers evidenced by a shift in their collapse mode. Rather than collapsing as a fluid by flowing into the sub-phase, a KL4 supported POPG monolayer instead collapses by folding. Furthermore, when KL4 is added to POPG containing monolayers they demonstrate an increased tolerance to repeated compression and expansion cycles while the opposite appears to be true for pure DPPC monolayers. We will discuss the potential role of electrostatic interactions in the rigidifying effect of KL4 on POPG containing monolayers in the context of the overall importance of collapse mode in establishing robust and reversible lipid monolayers. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D15.00009: Microrheology of protein layers at the air-water interface Myung Han Lee, Steven Cardinali, Daniel Reich, Kathleen Stebe, Robert Leheny Due to their amphiphilic nature, many proteins in aqueous solution will adsorb at the air-water interface to create a viscoelastic interfacial layer. We present an investigation of the formation and mechanical properties of interfacial protein layers formed by beta-lactoglobulin using microrheological techniques including multiple particle tracking and magnetic nanowire microrheology. We observe the interfacial rheology evolve in time through three stages: (i) an increase in viscosity, (ii) a period of spatial heterogeneity in which the interface contains elastic and viscous regions, and (iii) the development of a uniformly rigid elastic film. We evaluate the dependence of this evolution on the protein-protein interactions, which we tune by varying solution pH. As we will discuss, these studies illustrate the power of microrheological approaches to interfacial rheology. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D15.00010: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:30PM - 4:42PM |
D15.00011: Behavior of a polymer chain confined by a membrane Fabrice Thalmann, Carlos M. Marques We consider a single polymer chain, grafted on a flat and rigid subtrate, covered by a membrane. The membrane presents some affinity for the surface, caused by non specific adhesive interactions. The challenge is to anticipate and describe the different possible relative configurations of the membrane and the polymer chain, depending on parameters such as the surface tension and curvature of the membrane, or the chain gyration radius. We propose for this system a phase diagram of the different regimes, as well as quantitative predictions for comparison with some recent experiments. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D15.00012: Critical swelling of fluctuating capsules Haim Diamant, Emir Haleva In many natural transport processes the solute molecules to be transported are encapsulated in semipermeable, flexible membrane vesicles of micron size. We study the swelling of such fluctuating capsules, as the number of encapsulated particles is increased, or the concentration of the outer solution is decreased. The approach to the maximum volume-to-area ratio and the associated buildup of membrane tension involve a continuous phase transition and follow universal scaling laws. The criticality and its features are model-independent, arising solely from the interplay between volume and surface degrees of freedom.\footnote{E.\ Haleva and H.\ Diamant, Phys.\ Rev.\ Lett.\ {\bf 101}, 078104 (2008).} [Preview Abstract] |
Session D16: Focus Session: BEC-BCS Crossover
Sponsoring Units: DAMOPChair: Kathy Levin, University of Chicago
Room: 317
Monday, March 16, 2009 2:30PM - 3:06PM |
D16.00001: BCS to BEC evolution for mixtures of fermions with unequal masses Invited Speaker: I discuss the zero and finite temperature phase diagrams of a mixture of fermions with unequal masses with and without population imbalance, which may correspond for example to mixtures of $^6$Li and $^{40}$K, $^6$Li and $^{87}$Sr, or $^{40}$K and $^{87}$Sr in the context of ultracold atoms. At zero temperature and when excess fermions are present, at least three phases may occur as the interaction parameter is changed from the BCS to the BEC regime. These phases correspond to normal, phase separation, or superfluid with coexistence between paired and excess fermions. The zero temperature phase diagram of population imbalance versus interaction parameter presents a remarkable asymmetry between the cases involving excess lighter or heavier fermions [1, 2], in sharp contrast with the symmetric phase diagram corresponding to the case of equal masses. At finite temperatures, the phase separation region of the phase diagram competes with superfluid regions possessing gapless elementary excitations [3] for certain ranges of the interaction parameter depending on the mass ratio. Furthermore, a phase transition may take place between two superfluid phases which are topologically distinct. The precise location of such transition is sensitive to the mass ratio between the two species of fermions. Signatures of this possible topological transition are present in the momentum distribution or structure factor, which may be measured experimentally in time-of-flight or through Bragg scattering, respectively. Lastly, throughout the evolution from BCS to BEC, I discuss the critical current and sound velocity for unequal mass systems as a function of interaction parameter and mass ratio. These quantities may also be measured via the same techniques already used in mixtures of fermions with equal masses. \newline [1] M. Iskin, and C. A. R. Sa de Melo, Phys. Rev. Lett. 97, 100404 (2006). \newline [2] M. Iskin and C. A. R. Sa de Melo, Phys. Rev. A 76, 013601 (2007). \newline [3] Li Han, and C. A. R. Sa de Melo, arXiv:0812.xxxx [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D16.00002: Spin-imbalanced atomic Fermi gases in one dimension and the prospects for FFLO superconductivity C.J. Bolech, P. Kakashvili Growing expertise to engineer, manipulate and probe different analogs of condensed matter systems allows to probe properties of exotic pairing states such as the Fulde-Ferrell-Larkin- Ovchinnikov state. Inspired by ongoing experiments at Rice university, we are studying the pairing in spin-imbalanced ultracold atomic system of fermions in one dimension. Calculations are done using the Bethe Ansatz technique and the trap is incorporated into the solution via local density (Thomas-Fermi) approximation. The thermodynamic-Bethe-Ansatz equations are solved numerically and different density profiles (total-, spin- and entropy-densities) are calculated in the trap for different finite temperatures. A scheme to identify the phase diagram using total density profiles in the trap is proposed that would be immediately useful for experimentalists. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D16.00003: Finite temperature effects of $^{6}$Li-$^{40}$K mixtures in the BCS-BEC crossover Hao Guo, Chihchun Chien, Yan He, Qijin Chen, Kathryn Levin Recent experiments on mixtures of ultra-cold fermions of different species inspire study of pairing between fermions with different masses. We study systematically $^{6}$Li-$^{40}$K mixtures with tunable attractive interactions in the BCS-Bose Einstein condensation crossover. Pairing fluctuations which are important at finite temperatures are included in a consistent fasion. Population imbalance of the two species is also considered. We found an intermediate-temperature superfluid phase which is similar to the one found in polarized Fermi gases with equal mass. We present superfluid transition temperature for a broad range of polarization and interaction strength and analyze stability of possible superfluid phases. Phase diagrams at and near unitarity are presented. Polarized superfluids are shown to be stablized when the light species is the majority. Thus, in contrast to pairing between fermions with equal mass, observation of stable low-temperature polarized superfluids near unitarity is more feasible in $^{6}$Li-$^{40}$K mixtures. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D16.00004: Phenomenology Of Trapped Polarized Fermi Gases Leslie Baksmaty, Hong Lu, Han Pu, Carlos Bolech We discuss recent, apparently contradictory experimental results on trapped, polarized, resonantly interacting interacting fermions. Our analysis occurs in the context of a full 3D Bogoliubov-deGennes analysis and we dwell on the possible roles of the confining geometry in producing density distortions away from the local density approximation with implications for the Superfluid-Normal transition (Clogston limit). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D16.00005: Rotation induced superfluid-normal phase separation in trapped Fermi gases Menderes Iskin, Eite Tiesinga We use the Bogoliubov-de Gennes formalism to analyze the effects of rotation on the ground state phases of harmonically trapped Fermi gases, under the assumption that quantized vortices are not excited. We find that the rotation breaks Cooper pairs that are located near the trap edge, and that this leads to a phase separation between the nonrotating superfluid (fully paired) atoms located around the trap center and the rigidly rotating normal (nonpaired) atoms located towards the trap edge, with a coexistence (partially paired) region in between. Furthermore, we show that the rotation reveals a topological quantum phase transition: the superfluid phase that occurs in the coexistence region is characterized by a gapless excitation spectrum, and that it is distinct from the gapped phase that occurs near the trap center. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D16.00006: Pairing Instability in Two-Dimensional Rotating Fermion Liquids Near Unitarity Predrag Nikolic Fermionic superfluids can undergo phase transitions into different kinds of normal regimes, characterized by whether Cooper pairs remain locally stable. If the normal phase retains strong pairing fluctuations, it behaves like a liquid of vortices as seen in cuprate superconductors. We argue that analogous strongly correlated normal states exist in two-dimensional neutral fermion liquids near unitarity, where superfluid is destroyed by fast rotation. The formal analysis is based on a model with SP(2N) symmetry which describes the quantum critical region in the vicinity of a broad Feshbach resonance. Assuming that pairing is the only instability in perturbation theory, we map the universal phase diagram in two-dimensions. Such a pairing instability is driven by macroscopically degenerate collective modes, which makes the Abrikosov flux lattice of the superfluid particularly susceptible to quantum melting. Combining this observation with a renormnalization group analysis, we conclude that the unconventional normal states can be expected in the vicinity of the universal pairing instability, especially at low temperatures in the BCS limit. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D16.00007: Number of closed-channel molecules in the BEC-BCS crossover Felix Werner, Leticia Tarruell, Yvan Castin Using a two-channel model, we show that the number of closed-channel molecules in a two-component Fermi gas close to a Feshbach resonance is directly related to the derivative of the energy of the gas with respect to the inverse scattering length. We extract this quantity from the fixed-node Monte~Carlo equation of state and we compare to the number of closed-channel molecules measured in the Rice experiment with lithium [Partridge et al., Phys. Rev. Lett. 95, 020404 (2005)]. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D16.00008: Breathing mode frequencies of a rotating Fermi gas in the BCS-BEC crossover region Theja De Silva We study the breathing mode frequencies of a rotating Fermi gas trapped in a harmonic plus radial quartic potential. We find that as the radial anharmonicity increases, the lowest order radial mode frequency increases while the next lowest order radial mode frequency decreases. Then at a critical anharmonicity, these two modes merge and beyond this merge the cloud is unstable against the oscillations. The critical anharmonicity depends on both rotational frequency and the chemical potential. As a result of the large chemical potential in the BCS regime, even with a weak anharmonicity the lowest order mode frequency increases with decreasing the attractive interaction. For large enough anharmonicities in the weak coupling BCS limit, we find that the excitation of the breathing mode frequencies make the atomic cloud unstable. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D16.00009: Superfluid excitations of dipolar fermi gases Piotr Deuar, Mikhail Baranov, Georgy Shlyapnikov The collective and single-particle excitations of a gas of fermionic dipoles have been determined for the case of a uniform, single-species, fully polarised gas below superfluid (BCS) critical temperature. Its behaviour, especially damping, differs strongly from the s-wave BCS gas due to a node line in its quasiparticle excitation spectrum that resembles that in the hypothetical polar phase of He-3 and exotic superconductors. One finds: (1) Anisotropic damping of collective modes even at T=0. (2) An ``aligned superfluid'' regime with no analogue in the s-wave-interacting gas, for excitations with energy well below kT. Here good quality superfluidity occurs only in directions concentrated broadly around the polarisation, whereas other directions are strongly damped. (3) Current response to external forcing of the gap is anisotropic and at an angle to the applied probe. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D16.00010: Two-fluid hydrodynamic modes in a strongly interacting Fermi gas Edward Taylor, Hui Hu, Xia-Ji Liu, Sandro Stringari, Allan Griffin Landau's theory of two-fluid hydrodynamics provides an exact description of the low-energy dynamics of all strongly interacting superfluids described by a 2-component order parameter. Extending our recent work, we report on improved variational solutions of the two-fluid hydrodynamic modes in trapped two-component Fermi gases close to unitarity. We show that the two-fluid mode frequencies are identical to the predictions of regular (Euler) hydrodynamics except at certain temperatures where these in-phase modes strongly hybridize with out-of-phase modes. Although two-fluid hydrodynamic modes have been extensively studied in superfluid helium, experiments in trapped quantum gases have yet to detect a clear signal of these modes. We discuss the reasons for this and suggest several experimental signatures of two-fluid behaviour in trapped Fermi superfluids. Measuring the two-fluid mode frequencies is a promising way of testing microscopic theories of the thermodynamic and transport properties at unitarity. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D16.00011: Adiabatic Dynamics of the Superconducting Order Parameter Minxi Jiang, Qian Niu From the time-dependent variational principle and taking into Berry phase effects, we formulate the dynamics of superconducting order parameter in the region where it evolves much slower compared to the timescale of quasi-particles. Collective mode in this region is calculated and compared with previous result obtained from the random-phase approximation which is valid in the opposite limit. We discuss applications to BCS/BEC states of the quantum Fermi gases. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D16.00012: The Efimov Effect and Color Superconductivity in a Three-State Fermi Gas J.R. Williams, J.H. Huckans, E.L. Hazlett, R.W. Stites, Y. Zhang, K.M. O'Hara We have created a quantum degenerate $^6$Li gas with equal populations in the three lowest energy hyperfine states. This three-state Fermi gas is stable against two-body inelastic collisions but decays by three-body recombination. We measure the rate of three-body recombination which can be used as a signature of the Efimov effect and which determines whether conditions are favorable for BCS pairing. The three pairwise $s$-wave scattering lengths exhibit overlapping Feshbach resonances at 690, 810 and 834 Gauss. As we vary the field between 0 and 834 Gauss, we find that the three-body recombination rate constant varies by over four orders of magnitude. High stability is achieved near 0 and 570 Gauss. We observe narrow resonant loss features near 130 and 500 Gauss. Recent calculations indicate that these resonant features arise from Efimov trimer states near threshold[1]. We also report on the rate of three-body recombination between 834 and 2000 Gauss. Our determination of the three-body parameters in this regime will guide future experiments aimed at achieving color superconductivity in this system.\\ \\ E. Braaten, H.-W. Hammer, D. Kang, and L. Platter, arXiv:0806.0587. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D16.00013: Phase diagram, extended domain walls, and soft collective modes in a three-component fermionic superfluid Gianluigi Catelani, Emil Yuzbashyan We study the phase diagram of a three-component Fermi gas with weak attractive interactions, which shows three superfluid and one normal phases. At weak symmetry breaking between the components the existence of domain walls interpolating between two superfluids introduces a new length scale much larger than the coherence length of each superfluid. This, in particular, limits the applicability of the local density approximation in the trapped case, which we also discuss. In the same regime the system hosts soft collective modes with a mass much smaller than the energy gaps of individual superfluids. We derive their dispersion relations at zero and finite temperatures and demonstrate that their presence leads to a significant enhancement of fluctuations near the superfluid-normal transitions. [Preview Abstract] |
Session D17: Focus Session: Foundations of Quantum Theory
Sponsoring Units: GQIChair: Ian Durham, Saint Anselm College
Room: 318
Monday, March 16, 2009 2:30PM - 3:06PM |
D17.00001: LeRoy Apker Award Talk: Factoring Quantum Logic Gates with Cartan Involutions Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D17.00002: Types and location of information Looi Shiang Yong, Vlad Gheorghiu, Robert B. Griffiths Imagine having some quantum information encoded in $n$ carrier qubits. We are interested in the question of how much information is present in a subset of the carrier qubits. In the case where the encoding is done using a stabilizer code, we have a precise and complete answer. The two extreme cases of having too small a subset whereby no information is present versus having a large subset of almost $n$ qubits from which all the information can be extracted are already well understood. In this talk we focus on the intermediate situation where only partial information is present. For this purpose we define different ``types'' of information, where the presence of a type of information on a subset of carrier qubits implies the ability to distinguish a particular set of encoded states associated to that type. With this we can determine how much and what types of information are present in any given subset of carrier qubits. With the help of some simple examples, we will show how sometimes only ``classical'' information is present and sometimes more can be present. Finally our results can be generalized to higher dimensional qudit stabilizer codes. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D17.00003: Testing Born's rule in Quantum Mechanics using a Triple slit experiment Urbasi Sinha, Christophe Couteau, Zachari Medendorp, Immo Soellner, Raymond Laflamme, Rafael Sorkin, Gregor Weihs In Mod. Phys. Lett.A \textbf{9} 3119 (1994), one of us (R.D.S) investigated a formulation of quantum mechanics as a generalized measure theory. Quantum mechanics computes probabilities from the absolute squares of complex amplitudes, and the resulting interference violates the (Kolmogorov) sum rule expressing the additivity of probabilities of mutually exclusive events. However, there is a higher order sum rule that quantum mechanics does obey, involving the probabilities of three mutually exclusive possibilities. We could imagine a yet more general theory by assuming that it violates the next higher sum rule. In this presentation, we report results from an ongoing experiment that sets out to test the validity of this second sum rule by measuring the interference patterns produced by three slits and all the possible combinations of those slits being open or closed. We use either attenuated laser light or a heralded single photon source (using parametric down conversion) combined with single photon counting to confirm the single photon character of the measured light. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D17.00004: Cartan Involutions in Quantum Information Peter Love We discuss some applications of Cartan decompositions and the corresponding involutions of the unitary group in quantum information theory. Recently, such involutions were used to obtain a constructive quantum Shannon decomposition with an application to quantum circuits. We will discuss some practical aspects of the use of this decomposition to obtain circuits for arbitrary unitary matrices. We discuss further applications of these techniques to the computation of mixed state entanglement and the parameterization of quantum operations on open systems. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D17.00005: Quantum Foundations of Quantum Information Invited Speaker: The main foundational issue for quantum information is: What is quantum information about? What does it refer to? Classical information typically refers to physical properties, and since classical is a subset of quantum information (assuming the world is quantum mechanical), quantum information should--and, it will be argued, does--refer to quantum physical properties represented by projectors on appropriate subspaces of a quantum Hilbert space. All sorts of microscopic and macroscopic properties, not just measurement outcomes, can be represented in this way, and are thus a proper subject of quantum information. The Stern-Gerlach experiment illustrates this. When properties are compatible, which is to say their projectors commute, Shannon's classical information theory based on statistical correlations extends without difficulty or change to the quantum case. When projectors do not commute, giving rise to characteristic quantum effects, a foundation for the subject can still be constructed by replacing the ``measurement and wave-function collapse'' found in textbooks--an efficient calculational tool, but one giving rise to numerous conceptual difficulties--with a fully consistent and paradox free stochastic formulation of standard quantum mechanics. This formulation is particularly helpful in that it contains no nonlocal superluminal influences; the reason the latter carry no information is that they do not exist. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D17.00006: Measuring the distance between unitary propagators of quantum systems of differing dimensions Matthew Grace, Jason Dominy, Robert Kosut, Constantin Brif, Herschel Rabitz In this work, we develop a general distance measure that evaluates the distance between unitary quantum operations of differing dimensions which is (i) independent of the initial state of the system, (ii) straightforward to numerically calculate, and, most importantly, (iii) designed to directly evaluate quantum operations resulting from open-system dynamics. This measure is a natural extension of distance and corresponding fidelity measures employed in previous works that construct closed-system unitary operations. The properties of this measure are desirable for the calculation of distance, e.g., optimal control applied to open systems for quantum information processing, and enable a consistent comparison of quantum operations resulting from both closed- and open-system dynamics. As a numerical example, this distance measure is used to evaluate the fidelity of quantum operations resulting from the optimal control of one- and two-qubit unitary operations in the presence of a decohering environment. This example illustrates the utility of this measure for use in designing unitary quantum operations from open-system dynamics. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D17.00007: Riemannian Curvature in Quantum Computational Geometry Howard Brandt In the Riemannian geometry of quantum computation [1]-[3], the quantum evolution is described in terms of the special unitary group of n-qubit unitary operators with unit determinant. To elaborate on one aspect of the methodology, the Riemannian curvature on the group manifold is explicitly derived using the associated Lie algebra. This is important for investigations of the global characteristics of geodesic paths in the group manifold. [1] M. R. Dowling and M. A. Nielsen, ``The Geometry of Quantum Computation,'' \textit{Quantum Information and Computation} \textbf{8}, 0861-0899 (2008). [2] H. E. Brandt, ``Riemannian Geometry of Quantum Computation,'' to appear in \textit{Nonlinear Analysis} (2008). [3] H. E. Brandt, ``Riemannian Geometry of Quantum Computation,'' AMS Short Course Lecture, to appear in Proc. Symposia in Applied Mathematics., American Mathematical Society (2009). [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D17.00008: Closed timelike curves enable perfect state distinguishability Todd A. Brun, Jim Harrington, Mark M. Wilde The causal self-consistency condition for closed timelike curves can give rise to nonlinear interactions on chronology-respecting qubits. We demonstrate that particular unitary interactions between closed timelike curve qubits and chronology-respecting qubits allow perfect distinguishability of nonorthogonal states, and provide a constructive proof for an arbitrary number of nonorthogonal states. This has a number of highly significant consequences. For example, an adversary with access to closed timelike curves can break the B92, BB84, and SARG04 quantum key distribution protocols, or any prepare-and-measure quantum key distribution scheme. Our result also implies that a party with access to closed timelike curves can violate the Holevo bound by accessing more than $\log(N)$ bits of information from an $N$-dimensional quantum state. In principle, he can transmit an arbitrarily large amount of classical information with a quantum system of fixed size. We discuss the implications of this for quantum cloning. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D17.00009: Experimental Basis for IED Particle Model J. Zheng-Johansson The internally electrodynamic (IED) particle model is built on three experimental facts: a) electric charges present in all matter particles, b) an accelerated charge generates electromagnetic (EM) waves by Maxwell's equations and Planck energy equation, and c) source motion gives Doppler effect. A set of well-kwon basic particle equations have been predicted based on first-principles solutions for IED particle (e.g. J Phys CS{\bf 128}, 012019, 2008); the equations are long experimentally validated. A critical review of the key experiments suggests that the IED process underlies these equations not just sufficiently but also necessarily. E.g.: 1) A free IED electron solution is a plane wave $\psi \dot{=} Ce^{i(k_d X-\omega T)}$ requisite for producing the diffraction fringe in a Davisson-Germer experiment, and of also all basic point-like attributes facilitated by a linear momentum $\hbar k_d $ and the model structure. It needs not further be a wave packet which produces not a diffraction fringe. 2)The radial partial EM waves, hence the total $\psi$, of an IED electron will, on both EM theory and experiment basis -not by assumption, enter two slits at the {\it same} time, as is requisite for an electron to interfere with itself as shown in double slit experiments. 3) On annihilation, an electron converts (from mass $m$) to a radiation energy $\hbar \omega$ without an acceleration which is externally observable and yet requisite by EM theory. So a charge oscillation of frequency $\omega$ and its EM waves must regularly present internal of a normal electron, whence the IED model. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D17.00010: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 5:18PM - 5:30PM |
D17.00011: Quantum entanglement and informational activities of biomolecules Hanan Al-Shargi, Simon Berkovich Our model of holographic Universe [1] explains the surprising property of quantum entanglement and reveals its biological implications. The suggested holographic mechanism handles 2D slices of the physical world as a whole. Fitting this simple holistic process in the Procrustean bed of individual particles interactions leads to intricacies of quantum theory with an unintelligible protrusion of distant correlations. Holographic medium imposes dependence of quantum effects on absolute positioning. Testing this prediction for a non-exponential radioactive decay could resolutely point to outside ``memory.'' The essence of Life is in the sophistication of macromolecules. Distinctions in biological information processing of nucleotides in DNA and amino acids in proteins are related to entropies of their structures. Randomness of genetic configurations as exposed by their maximal entropy is characteristic of passive identification rather than active storage functionality. Structural redundancy of proteins shows their operability, of which different foldings of prions is most indicative. Folding of one prion can reshape another prion without a direct contact appearing like ``quantum entanglement,'' or ``teleportation.'' Testing the surmised influence of absolute orientation on the prion reshaping can uncover the latency effects in the ``mad cow'' disease. 1. Simon Berkovich, TR-GWU-CS-07-006, http://www.cs.gwu.edu/research/reports.php [Preview Abstract] |
Session D18: Focus Session: Transport and Optical Properties of Conjugated Polymers and other Solution Processable Semiconductors
Sponsoring Units: DPOLYChair: Anvar Zakhidov, University of Texas at Dallas
Room: 319
Monday, March 16, 2009 2:30PM - 2:42PM |
D18.00001: Microstructure and Charge Transport in pBTTT Thin Film Transistors Chenchen Wang, Alberto Salleo, Ludwig Goris, Iain McCulloch, Martin Heeney, Alexander Ziegler The present work focused on the morphology and charge carrier mobility of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT) films. In annealed pBTTT films on oxide functionalized with octyltrichlorosilane (OTS), TEM study shows that the large-scale terraces observed by AFM, which was believed to be the reason for high charge carrier mobility, are composed of smaller crystalline grains. Using the mobility edge model, we find that, compared with the film on oxide, the density of trap states at the band edge is reduced in the film on OTS, and it is about the same as the trap density in poly(3-hexylthiophene) (P3HT), which has lower carrier mobility. This result indicates that the higher room-temperature mobility of pBTTT (0.34 cm$^{2}$/Vs) compared to P3HT (0.02 cm$^{2}$/Vs) is due to a high quasi-free carrier mobility and not to a lower trap density as previously thought. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D18.00002: Interfacial Charge Transfer in Nanoscale Polymer Transistors Jeffrey Worne, Rajiv Giridharagopal, Kevin Kelly, Douglas Natelson, John Anthony Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred in a number of photoemission experiments. We present electronic transport measurements in very short channel (L < 100 nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact resistance and the zero gate voltage conductance are consistent with such charge transfer. Short channel conduction in devices with Pt contacts is greatly enhanced compared to analogous devices with Au contacts, consistent with charge transfer expectations. Alternating current scanning tunneling microscopy (ACSTM) provides further evidence that holes are transferred from Pt into P3HT, while much less charge transfer takes place at the Au/P3HT interface. We have also begun to use these same techniques to investigate the nature of interfacial charge transfer between metal electrodes and pentacene. We use these data together with our previous results to develop a more complete picture of metal/organic interfaces. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D18.00003: Ab initio study of molecular packing of organic semiconducting materials Sefa Dag, Lin-Wang Wang The self-organizing and electronic properties of organic semiconducting material, poly(3-hexylthiophene) (P3HT), have been investigated in terms of Ab initio density functional calculations. We found that thiophene-thiophene interaction in adjacent layers has a strong influence to create stacked planar structures. Our calculations showed that P3HT chains tend to stack into planar structures, in which adjacent thiophene-thiophene rings along the stacking direction are $180^o$ rotated with respect to each other. Theoretical powder diffraction profile of this structure showed same structure with experimental reflection peaks. We also showed enhanced transport resulting from the organization of P3HT chains. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D18.00004: Electrical Noise in Individual Conducting Polymer Nanowires Alexey Kovalev, Yanyan Cao, Theresa Mayer, Thomas Mallouk Electrical property characterizations of conducting polymer nanostructures have been limited primarily to resistance measurements. Electrical noise is one aspect that is usually overlooked, yet critical to their device performance. Moreover, electrical noise is more sensitive to the polymer doping and microstructure than resistance, which makes it particularly interesting for sensor applications. In this talk, we will present the results on the electrical noise measurements of individual multisegmented electrodeposited nanowires based on Poly(3,4-ethylenedioxythiophene (PEDOT) [1]. The polymer was electrochemically doped with either poly(4-styrenesulfonic acid) (PSS) or perchlorate (ClO$_{4})$. The nanowires had gold contacts on both ends and were measured in four-point and two-point configurations. We found that the electrical noise behavior is typical of $1/f$ noise, with a spectral density that depends on the polymer structure and is affected by the ambient conditions. Our data show that the contact noise represents a significant contribution to the total noise level. We will discuss the interpretation of these results assuming that the polymer is a disordered conductor. [1] Cao \textit{et al.}, \textit{Nano Letters} \textbf{Article ASAP} [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D18.00005: Thin film morphology of organic electronic materials S. D. Hudson, R. J. Kline, D. M. Delongchamp, O. D. Jurchescu, D. J. Gundlach, L. J. Richter The crystal orientation and morphology of a polythiophene (pBTTT) and an anthradithiophene (diF-TEADT, a pentacene analog) in thin films have been explored by TEM, SEM, AFM, GISAXD, NEXAFS, polarized FTIR and ellipsometry. The orientation has a striking influence on the performance of thin film transistors. We show that solution casting and annealing conditions have a significant effect on the morphology of pBTTT. Correlations between film surface step morphology and crystal orientation are determined. Interfacial interactions with the substrate (gold, silica, or fluorinated sam) govern the crystal orientation and crystal aggregate morphology of diF-TESADT. Depending on this orientation, the carrier mobility spans from approximately 0.001 cm$^{2}$/Vs to 0.4 cm$^{2}$/Vs. Epitaxial relationships within crystal aggregates are observed. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D18.00006: Conjugated polymer/layered inorganic nanocomposites: solution processable route to enhanced thermoelectric performance Kevin See, Jeffrey Urban, Rachel Segalman In recent years, incorporation of nanostructuring has led to notable improvements in the performance of thermoelectric materials. At a given temperature T, the thermoelectric figure of merit ZT is given by $\frac{S^2\sigma T}{\kappa }$ , where S is the Seebeck coefficient, $\sigma $ the electrical conductivity and $\kappa $ the thermal conductivity. In most cases, improvement in ZT through nanostructuring has been realized via reduction in thermal conductivity $\kappa $ rather than increases in the power factor S$^{2}\sigma $. Here we utilize solution-based intercalation chemistry to create layered inorganic/conjugated polymer nanocomposites with designed nanoscale interfaces engineered to enhance the power factor by energy filtering. The layered inorganic material Sb$_{2}$Te$_{3}$ was intercalated with poly(3-hexylthiophene), and the resulting composite material was cast into thin films from solution. The resulting devices exhibit Seebeck coefficients with two-fold enhancement over those reported for bulk Sb$_{2}$Te$_{3 }$with known conductivities for solution-processed films. These results demonstrate the promise of these novel intercalated materials for high performance solution processable thermoelectric materials. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D18.00007: Revealing Transmission in Metal-Molecule Junctions Using Length Dependant Thermopower Measurements Jonathan A. Malen, Peter Doak, Kanhayalal Baheti, T. Don Tilley, Arun Majumdar, Rachel A. Segalman Conductance in metal-molecule junctions is known to trend with molecular endgroups, backbone, and length, but a more complete picture of the junction's transmission structure has been hitherto elusive. We now report complimentary trends in the junction's thermopower ($S)$ that reveal length dependent changes in molecular orbital alignment and coupling with contact states. Phenylenediamines, phenylenedithiols, and alkanedithiols trapped between gold contacts were examined. $S$ increases linearly with length for phenylenediames and dithiols while it decreases linearly in alkanedithiols. Comparison of this data suggests that the molecular backbone determines the length dependence of $S$, while the endgroup determines the zero-length, or contact $S$. Transport in phenylenes was dominated by the HOMO, which moves closer to the Fermi energy of the contacts as \textit{$\sim $1/L}, and broadens due to contact coupling as \textit{$\sim $e}$^{-L}$. In contrast, the decreasing trend in $S$ for alkanedithiols suggests that transmission is largely effected by gold-thiol gap states between the HOMO and LUMO. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D18.00008: Thermally-Induced Mesophase Transitions in Alkyl-Substituted Thienoacenes Charles M. Shaw, Xinnan Zhang, Lidaris San Miguel Rivera, Geetha G. Nair, Antal Jakli, Adam J. Matzger, David C. Martin Pentathienoacene (T$_{5}$) is an organic molecule---first synthesized in recent years---that is most succinctly described as the thiophene analog of pentacene. In this study, the solid-state structure and phase behavior of dioctyl- and didodecyl-substituted T$_{5}$ were examined \emph{via} differential scanning calorimetry (DSC), variable-temperature, polarized optical microscopy (VT-POM), variable-temperature X-ray diffraction (VT-XRD) and electron diffraction (ED). DSC reveals the presence of a number of phase transitions, while ED, VT-OM and VT-XRD reveal the details of the structural changes of these transitions. The first phase transition exhibited by both materials is a crystal--crystal transformation that involves the contraction of the unit cell along the long axis by nearly 25\%. This change has been attributed to the introduction of numerous gauche defects at elevated temperatures. Further heating causes both molecules to exhibit a Smectic C liquid crystalline phase, identified by VT-OM and VT-XRD. VT-XRD was also utilized to elucidate lattice parameters for these various phases. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D18.00009: Fullerene-based anchoring groups for molecular electronics Christian Martin, Dapeng Ding, Jakob Sorensen, Thomas Bjornholm, Jan van Ruitenbeek, Herre van der Zant We present results on a new fullerene-based anchoring group for molecular electronics. Using lithographic mechanically controllable break junctions in vacuum and at RT we have studied the electrical properties of 1,4-bis(fullero[c]pyrrolidin-1-yl)benzene. The compound can be self-assembled from solution and forms molecular junctions with a low-bias conductance of $3 \cdot 10^{-4}$ G$_0$. Compared to 1,4-benzenedithiol it exhibits a considerably lower conductance spread. Statistical analyses of the breaking process confirm the stability of the fullerene-gold bond. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D18.00010: An organic nanoparticles transistor behaving as a spiking synapse Dominique Vuillaume, Fabien Alibart, Christophe Novembre, David Guerin, Stephane Pleutin, Kamal Lmimouni, Christian Gamrat We demonstrate that an organic transistor, made of metal nanoparticles (NP) embedded into an organic semiconductor channel, behaves as a spiking synapse. We demonstrate that this device exhibits the main behavior of a biological synapse. For instance, it can be programmed to work as an excitatory or inhibitory synapse; it exhibits shot-term plasticity as well as spike timing dependent plasticity. This behavior is obtained by virtue of the combination of two properties: the transconductance gain of the transistor and the memory effect due to charges stored in the NP. The gold NP are immobilized into the source-drain channel by using surface chemistry (self-assembled monolayers) and they were subsequently covered by a thin film of pentacene. In a biological synapse, the excitatory behavior means that an incoming signal with a given frequency and duty cycle induces a post-synaptic signal having an increasing trend, whereas in the case of an inhibitory synapse, the post-synaptic signal tends to decrease. This behavior is exactly what we demonstrated for the ONTS. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D18.00011: The role of the oxygen/water redox couple in suppressing electron conduction in field-effect transistors Pierre L. Levesque, Carla M. Aguirre, Matthieu Paillet, Fran\c{c}ois Lapointe, Benoit C. St-Antoine, Patrick Desjardins, Richard Martel Much like with organic semiconductor FETs, a characteristic of carbon nanotube based devices has been their almost exclusive p-type character in air. Electron transport can be observed only under certain conditions, for instance devices annealed in vacuum. We investigated the impact of the chemical nature of the substrate and of ambient adsorbates on the field-effect switching behavior of both nanoscale and thin-film FETs. Our study, using carbon nanotubes as the testbed, revealed that the intrinsic material properties are modified when an adsorbed water layer containing solvated oxygen is present on the SiO$_{2}$ surface and lead to the reduction of n-type conduction. This finding demonstrates that an electrochemical charge transfer reaction between the semiconducting channel and the aqueous oxygen redox couple is the underlying phenomenon governing the suppression of electron conduction in these devices. This effect should be considered when measuring the transport properties of nanostructures such as nanowires, organic materials, nanotubes, graphene conducted on SiO$_{2}$/Si substrates. [Preview Abstract] |
Session D19: Focus Session: Grazing Incidence Scattering and New Imaging Techniques
Sponsoring Units: DPOLYChair: Marcus Cicerone, National Institute of Standards and Technology
Room: 320
Monday, March 16, 2009 2:30PM - 3:06PM |
D19.00001: Probing kinetics and dynamics of nanocomposites with grazing-incidence small-angle x-ray scattering. Invited Speaker: Synthesizing complex nanocomposites and superstructures is of great interest in all areas of materials science and involving biology, chemistry, physics and engineering applications such as the fabrication of novel electronic, magnetic, and photonic devices. Since the entire synthesis and assembly process can take place far from equilibrium conditions, a controlled process has to be guided by a thorough understanding of the kinetics and dynamics in the composites. This requires measurement of the structure in situ and in real time with subnanometer spatial resolution and millisecond to subsecond temporal resolution. As an increasingly important structural-characterization technique, grazing-incidence small-angle x-ray scattering (GISAXS) finds vast applications in the research of nanostructures and nanocomposites at surfaces and interfaces. Most significantly, as a complementary method to conventional surface-sensitive tools such as scanning probe microscopy and electron microscopy, GISAXS can be used in situ and in real time to monitor the formation of the nanostructure or nanocomposite, which makes it most suitable for studying the kinetics of nanoassembly processes. The GISAXS technique can also be an integral part of numerous research, for example, those involving kinetics of mesoscaled ordered block copolymer thin films, kinetics of sol-gel processes, quantum dots, nanoparticles in ultrathin films, and dynamics and phase transitions 2D nanocrystal superlattices. Here, I will focus on the applications of GISAXS in real-time structure characterization, the dynamics in polymer/nanoparticle nanocomposies, and the challenges to elucidate nanostructure formation in nanoscience and nanotechnology. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D19.00002: Single beam approach for GISAXS Byeongdu Lee, Chieh-Tsung Lo, Pappannan Thiyagarajan, Zhongwei Niu, Qian Wang The multiple scattering effects present in the grazing incidence small-angle x-ray scattering (GISAXS) data are addressed theoretically as well as experimentally with measurement of a series of patterns at different incident angles, referred to as ``incident-angle-resolved GISAXS'' (IAR-GISAXS). We found that under certain conditions, it is possible to extract the correct structural features of the materials from the GISAXS data using the kinematic SAXS formalisms assuming a single beam, without the need to use the distorted wave Born approximation (DWBA) to account for the scattering by the reflected beam. Furthermore, the Kiessig fringes in GISAXS enable the measurement of average distance between the particle and the substrate, similar to the measurement of film thickness using the fringes in the x-ray reflectivity data. We believe that the methods developed here will expand the application of GISAXS as they enable the use of model-independent and kinematic SAXS theories to nanostructured 2D-ordered films. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D19.00003: Surface Dynamics of Free PS Chains on Chemically Identical Polymer Brushes: An XPCS Study Gokce Ugur, Bulent Akgun, Zhang Jiang, Suresh Narayanan, William J. Brittain, Mark D. Foster We found no relaxation of fluctuations of the brush surfaces within the range of time (0.2 -1100 s) and length scale (0.6-3 um) studied by X-ray photon correlation spectroscopy(XPCS). This is true for PS brushes of thicknesses of 9 - 101 nm and grafting density of 0.12-0.6 chains/nm$^{2}$ at temperatures up to 130C above bulk T$_{g}$. Results on the dynamics of a layer of untethered 2.2k PS chains on top of a PS brush surface show that placing the PS chains atop the brush dramatically slows down the surface relaxations of the film surface. As the ratio of the thickness of the layer of untethered chains to the thickness of the highly dense brush drops below $\sim $0.5, the surface relaxations become too slow to be observed readily with XPCS. Reducing grafting density of the underlying brush markedly slows the surface dynamics. The surface dynamics of the layer of ``free'' PS chains are coupled with those of the underlying brush. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D19.00004: X-ray Standing Wave Studies of Stability and Dynamics in Poly(4-bromostyrene)/Poly(4-vinylpyridine) Thin Films Yan Sun, Kenneth Shull, Jin Wang The thermodynamic stability and wetting behavior in systems consisting of two or three distinct layers of polymeric thin films have been investigated with atomic force microscopy (AFM) and x-ray standing waves (XSW) generated via total external reflection from an x-ray mirror. We have probed the structural evolution of thin poly(4-bromostyrene) (PBrS) films with various degrees of bromination, prepared on top of a poly(4-vinylpyridine) (P4VP) layer whose dynamics is influenced by its interaction with the underlying substrate and couples to that of PBrS. The addition of a top poly(styrene) (PS) layer was also used in some cases. The samples were subjected to annealing treatments above the polymer glass transition temperatures. Reflectivity and x-ray fluorescence from bromine markers in the PBrS layer were tracked. Dewetting of the PS occurred with sufficient annealing time, though the results suggest that this proceeded faster with low PBrS bromination. AFM studies on the PBrS/P4VP system revealed a clear PBrS thickness dependence on the dewetting morphology and dynamics of this layer. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D19.00005: Quantitative electron tomography and its application to polymer nanostructures Invited Speaker: The transmission electron microtomography (TEMT) is a powerful tool to visualize three-dimensional (3D) structures in many fields of materials science. Recently, researchers are trying not only to visualize 3D nano-structures but also to quantify them in order to seek a possible correlation between the 3D structures and materials' properties. However, one of the serious problems that prohibit TEMT from truly quantitative 3D images is the ``missing wedge'' in the Fourier space that is caused by the limitation of angular range available in transmission electron microscopes (TEM). Please note that the computerized tomography (CT), on which TEMT is based, requires projections from entire tilt angles, i.e. $\pm$90$^{\circ}$. Thus, the most faithful tactics for the CT is to tilt specimen over $\pm$90$^{\circ}$. In order to realize such requirement, a rod-shaped ZrO$_2$/polymer nano-composite whose diameter is ca. 150 nm was attached at the tip of a specially modified specimen holder without any supporting film. A complete set of tomograms has been generated for the first time from the 181 projections that were taken over the angular range of $\pm$90$^{\circ}$. One of the structural parameters characterizing the nano-composite, a volume fraction of ZrO$_2$, $\varphi$, was measured as a function of the maximum tilt angle, $\alpha$. It was found that $\varphi$ was in excellent agreement with the known volume fraction of ZrO2 when $\alpha$=90$^{\circ}$, i.e., $\pm$90$^{\circ}$ tilt, while $\varphi$ increased with decreasing $\alpha$. When $\alpha$=60$^{\circ}$ that is a typical maximum tilt angle, the measured $\varphi$ was larger by 20$\sim$30\% than the true value. In addition to the above TEMT experimental technique, some applications of TEMT to polymer nano-structures will be presented at the conference time. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D19.00006: Three-dimensional subwavelength imaging with phase-less power extinction tomography. Alexander A. Govyadinov, George Y. Panasyuk, John C. Schotland Modern near-field methods extend the spatial resolution of optical microscopes beyond the classical diffraction limit. However the majority of these methods only recover two-dimensional maps of optical intensity near the sample surface. The interpretation of these maps for manifestly inhomogeneous samples has been proven to be problematic. Here we derive an analytical technique which allows unique subwavelength 3D reconstruction of both real and imaginary parts of susceptibility of an inhomogeneous sample. Our technique is based upon the solution to the linearized near-field inverse scattering problem arising in the sample -- near-field tip system. The proposed approach requires neither phase measurements nor control over the phase of illuminating fields. The reconstruction is based on simple measurements of the power extinguished from illuminating waves in the total internal reflection mode and is intrinsically nondestructive. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D19.00007: Helium ion microscopy and its application to organic materials Steven Hudson, Andras Vladar, Bin Ming Helium ion microscopy (HeIM) is a new scanning probe microscopy that uses a He$^{+}$ ion beam. This microscope has improved resolution and depth of field in comparison to SEM, as demonstrated through imaging of metal particles. Organic materials, including patterned polyelectrolyte multilayers and organic semiconductor crystals, have also been imaged. The surface sensitivity, image contrast and qualitative secondary electron yield have been evaluated, in an effort to understand beam/specimen interactions and compare them with electron beam/sample interactions. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D19.00008: Robust Tips for High Resolution Chemical Imaging Carlos Barrios, Andrey Malkovskiy, Alexander Kisliuk, Alexei Sokolov, Mark Foster Tip enhanced Raman spectroscopy (TERS) combines scanning probe microscopy with Raman spectroscopy, taking advantage of apertureless near-field optics. A plasmonic structure at the apex of a sharp tip provides signal amplification required for chemical imaging. Plasmonic structure characteristics such as roughness, shape, and radius determine the spatial resolution and signal enhancement. Unfortunately, noble metal nanostructures have limited lifetimes due to mechanical, chemical, and thermal degradation. Lifetime extension requires slowing degradation processes while minimizing unfavorable influences on the optical response. An ultrathin SiO$_{x}$ protective coating provides lifetime improvement of silver plasmonic nanostructures on SPM tips. Controlled physical vapor deposition (PVD) of Al can be used to create ultrathin ($\sim $2-3 nm) Al$_{2}$O$_{3}$ coatings that improve significantly the stability and wear resistance of plasmonics structures without substantial degradation of optical properties. Such a coating completely prevented decay in plasmonic activity after 40 days of use. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D19.00009: Broadband CARS microscopy Marcus Cicerone, Joung Lee, Yeon Ho Kim, Sapun Parekh Coherent anti-Stokes Raman scattering (CARS) microscopy has exciting potential for rapid chemical imaging of materials and noninvasive imaging of biological systems, both in-vivo, and as these systems interact with materials. Although CARS is as much as 10$^{6}$ times more sensitive that spontaneous Raman scattering, it is accompanied by a nonresonant background (NRB) signal which can mask the resonant signal of interest. This background is generally of sufficient amplitude to make chemical imaging of biological systems difficult or impossible. We will present recent advances made in our lab, both experimental and in numerical data recovery, towards ameliorating the negative aspects of the NRB and facilitating non-invasive chemical microscopy for cell-material interactions. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D19.00010: Intensity Fluctuations of Optical Microscopy as a Means to Measure Axial Diffusion Malvika Bihari, Thomas Russell, David Hoagland Via optical microscopy, geometrically hindered motions of a single large solute (particle or polymer) can be imaged in real time. Here, intensity fluctuations of confocal fluorescence microscopy admit another way to probe such motions, one convenient when motions are perpendicular to a planar substrate. The focal plane is positioned within the substrate (lying on the microscope stage) and intensity fluctuations arise from motions in-and out- of the focal volume. Two experiments illustrate the new approach, diffusion within pores of a planar membrane or in solution near a solid wall. In the first, diffusion coefficients of spherical particles were measured inside pores of a track-etched polycarbonate membrane as functions of particle and pore size. In the second, anisotropic diffusion (perpendicular/parallel) of the same particles was measured within a few particle diameters of a solid boundary. Theory for hydrodynamically hindered diffusion in both cases is well developed, and data are compared to predictions. Two ways to assess particle/polymer motion, tracking single particles and correlating intensity fluctuations, will be discussed. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D19.00011: Three-Dimensional Imaging of Polymeric Nanostructures by Molecular Switching in Far Field Fluorescence Microscopy Chaitanya Ullal, Roman Schmidt, Alexander Egner, Benjamin Harke, Jan Keller, Douglas Adamson, Lars Kastrup, Stefan Hell Morphological studies of self assembled polymeric structures with length scales of interest below 100 nm have typically been conducted either by scattering-based techniques or electron and scanning probe microscopes. These techniques, however, do not provide easy access to truly 3D-structural information. In contrast, Far-field optical methods retain the advantage of simultaneously providing local, dynamic, and \textit{in situ} three-dimensional (3D) structural information. The diffraction limited resolution of its standard variants, however, restricts the minimum feature size that can be examined. We exploit molecular transitions of the fluorophores to circumvent the diffraction barrier and demonstrate the power of emerging far-field fluorescence microscopy with nanoscale resolution for the study of self-assembly. We simultaneously improve both the lateral ($x$,$y)$ and the axial ($z)$ resolution of stimulated emission depletion (STED) microscopy. The increased 3D resolution is used to unambiguously map the morphology of self assembled polymeric nanostructures in a facile manner. [Preview Abstract] |
Session D20: Polymers and Energy: Photovoltaics, Fuel Cells, Batteries II
Sponsoring Units: DPOLYChair: Azar Alizadeh, General Electric
Room: 321
Monday, March 16, 2009 2:30PM - 2:42PM |
D20.00001: Simulation study of proton transport in stretched nanocomposite ionomer fuel-cell membranes Philip Taylor, Elshad Allahyarov We have used coarse-grained simulation methods to investigate the effect of inclusions of nanoparticles on the stretching-induced structure orientation and on the proton conductivity of polymer electrolyte membranes. Uniaxial stretching of a Nafion film containing no inclusions causes a modest increase in proton conductivity in the direction of stretching. This effect does not persist to any significant degree after removal of the stretching stress. Stretching of a Nafion film containing spherical nanoparticles, on the other hand, causes a large increase in proton conductivity in the direction of stretching, and this effect persists to a much greater extent after the removal of the stretching stress. Simulations were performed with monodisperse nanoparticles whose diameters were in the range from 17 to 28 nm, and whose surfaces were either hydrophilic, neutral, or hydrophobic. The greatest effect in causing enhancement of the proton conductivity and in causing persistent ordering was found for hydrophilic nanoparticle inclusions of 28 nm diameter. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D20.00002: The role of nanoparticle-membrane coupling in nanocomposite ionomers Elshad Allahyarov, Philip Taylor Coarse-grained simulation methods have been used to investigate the effect of inclusions of spherical nanoparticles on the properties of Nafion$^{ \tiny \textregistered}$-like membranes. We find the clustering of the sulfonate head groups to be strongly affected by the presence of a monodisperse array of spheres when the sphere diameters lie in the range from 17 to 28 nm. This change in morphology enhances the proton conductivity of the membrane through the formation of channels connecting adjacent clusters. This effect was characterized in terms of the distribution of channel lengths of the hydrophilic phase. Simulations were performed for Nafion containing spherical nanoparticles whose surfaces were either hydrophilic, neutral (hard core), or hydrophobic. The diameters of the nanoparticles were changed while keeping fixed the volume fraction of inclusions. We find that the proton conductivity of these nanocomposites is always higher than the conductivity of ionomers without additives. This effect becomes most pronounced in nanocomposites containing particles whose surfaces are hydrophilic, and whose diameters are in the larger part of the range of sizes examined. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D20.00003: Multi-Lamellar Structures in Nafion Joseph A. Dura, Vivek S. Murthi, Michael R. Hartman, Sushil K. Satija, Charles F. Majkrzak Both proton conductivity and gas diffusion are key factors in the performance of a fuel cell proton exchange membrane, PEM. They are critically dependent on water content and morphology, especially in the three phase region where catalyst, PEM, and gases such as fuel or oxidizer co-exist. Here we show that lamellar structures composed of thin alternating water rich and Nafion rich layers exist at the interface between SiO$_{2}$ and the hydrated Nafion film. Lamellae thickness and number of layers increase with humidity. Some lamellae remained in the film after dehydration. Multilayer lamellae are not observed for Nafion on Au or Pt surfaces. Instead, a thin partially hydrated single interfacial layer occurs and decreases in thickness to a few angstroms as humidity is reduced to zero. The absorption isotherm of the rest of the Nafion film is similar to that of bulk Nafion for all three surfaces investigated. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D20.00004: RT-TDDFT simulation of the optical properties of a model organic photovoltaic device F. Vila, J.J. Rehr Organic solar cells are attracting much interest because of their potential as cost-effective photovoltaic devices. Prototypical cells consist of a bilayer of p- and n-type materials. The conversion of light into a current is initiated by the absorption of a photon in the p-type donor, mediated by the creation, diffusion and dissociation of an exciton, and finalized by a charge transfer to the n-type acceptor, with subsequent transport to the electrodes. To explore this issue, we simulate the optical response of a model bilayer cell composed of a polythiophene(pT)/C$_{60}$ donor/acceptor pair using an efficient implementation of real-time TDDFT.\footnote{Y. Takimoto \textit{et al.}, J. Chem. Phys. {\bf127}, 154114 (2007).} We find that the chain twist induced by the C$_{60}$ on the pT shifts the absorption onset from 1.8 to 2.0 eV. This shift is larger and of opposite sign compared to that induced by the inclusion of either regioregular or random side-chains in pT, and by the interaction between two pT chains. Finally, we discuss extensions for the simulation of charge transport and exciton mobility. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D20.00005: Collecting photo-generated charge carriers from metallo-organic materials A.R. Carter, J.H. Park, Y.-H. Chou, Y. Ghosh, C.R. Reed, L.M. Mier, T.L. Gustafson, M.H. Chisholm, A.J. Epstein Organic photovoltaic materials continue to garner attention as potential low cost and tunable alternatives to conventional inorganics. We report progress in utilizing hybrid metallo-organic materials that incorporate metal-metal (M-M) quadruply bonded units into oligothiophenes via carboxylate linkers.\footnote{ G. T. Burdzinski, \textit{et al.}, PNAS \textbf{105,} 15247 (2008).} Varying the metal (M = Mo, W) or the ligands shifts the energetics and can be exploited to extend absorption into the infrared. These materials have high absorbtivity from 300 nm (4.1 eV) to 900 nm (1.4 eV). We present the results of photophysical studies of structures that employ these materials. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D20.00006: Monolithic Tandem Organic Photovoltaic Cell Utilizing Transparent Carbon Nanotube Interlayer Kamil Mielczarek, Senku Tanaka, Raquel Ovalle Robles, Alexander Kuznetsov, Brian Wang, Dean Hsu, Ray Baughman, Anvar Zakhidov We demonstrate an organic photovoltaic multijunction cell in a monolithic parallel tandem structure in which transparent multi and single-walled nanotube sheets are used as an interlayer electrode connecting two cells; polymeric photovoltaic (PPV) cell or organic low molecular PV (OPV). Each cell is characterized independently and the short circuit current density of the tandem is shown to be larger than individual cells for the PPV-MWCNT-OPC tandem*. Overall efficiency is increased attributed to effective use of transparent CNTs and enhanced spectral sensitivity due to differing active layer materials.Computer model circuit simulation is used to analyze the parameters of cells in parallel and series configurations. Advantages of a parallel connection is shown for PV cells with differing photocurrents. The PPV-CNT-PPV and OPV-CNT-OPV cells are also created and described. *S.Tanaka, K Mielczarek, et.al., APL. (submitted 2008, October). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D20.00007: Tandems of solid dye-sensitized solar cell with carbon nanotubes interlayer Chao-Chen Yuan, Jiangbin Xia, Anvar Zakhidov The light-to-electron conversion efficiency of the dye-sensitized solar cell (DSC) was recently improved up to 11.1{\%}. However, this efficiency is not sufficient for cost-effective commercial production, so the expansion of the absorption region of the solar cell is needed. For transparent carbon nanotubes, parallel-connected tandem DSCs is developed. Novel parallel type of tandem cell structure is created. We create a parallel combination for cells using different dyes. The top cell is transparent and the bottom cell only uses light passing through the top cell. Instead of a common platinum counter electrode as interlayer, we use transparent carbon nanotubes (CNTs) coated on hole transport layers of each sub-cell, as an interlayer counter electrode. With high enough conductivity and high optical transparency, the compatibility of CNTs work as the interlayer counter electrode performing even better than Pt. The short-circuit current density (Jsc) for the tandem cell is demonstrated to be higher than that of separate the front and back photo electrodes.A model using light energy absorbed by the photo electrode is N719 top cell and a black-dye bottom cell is developed. Now the prototype of DSC tandem cell has been proved with the efficiency of 0.293{\%} with 0.2 cm$^{2}$ area. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D20.00008: NEXAFS Spectroscopy of Biomimetic Dyes for Solar Cells Peter Cook, Xiaosong Liu, Franz Himpsel Organic photovoltaics hold the potential for an inexpensive alternative to traditional silicon in solar cell production. A group of such dyes is investigated systematically including porphyrins, phthalocyanines, and cytochrome c, all of them characterized by a transition metal atom surrounded by a cage of four nitrogen atoms. X-ray absorption spectroscopy of the transition metal 2p and the nitrogen 1s absorption edges reveals the LUMO, the oxidation state of the transition metal, and its spin state. In addition, the sensitivity of these molecules to damage by photon-induced hot electrons is investigated. While the nitrogen cage is rather robust, the peptide bonds between the one hundred amino acids in cytochrome c are easily damaged. This finding suggests minimizing the size of biologically-inspired molecules for photovoltaic applications. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D20.00009: Semiconductor Conjugated Polymer-Quantum Dot Nanocomposites at the Air/Water Interface and Their Performance in Thin Film Solar Cells Zhiqun Lin, Matthew Goodman, Jun Xu, Jun Wang Organic-inorganic nanocomposites consisting of electroactive conjugated polymer, poly(3-hexylthiophene) (P3HT) intimately tethered on the surface of semiconductor CdSe quantum dot (i.e., P3HT-CdSe nanocomposites) at the air/water interface formed via Langmuir isotherms were explored for the first time. The P3HT-CdSe nanocomposites displayed a high pressure plateau in the Langmuir isotherm, illustrating their complex packing at the air/water interface. Furthermore, photovoltaic devices fabricated from the LB depositions of the P3HT-CdSe nanocomposites exhibited a relatively high short circuit current, $I_{SC}$, while maintaining a thin film profile. These studies provide insights into the fundamental behaviors of semiconductor organic-inorganic nanocomposites confined at the air/water interface as well as in the active layer of an organic-based photovoltaic device. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D20.00010: Effect of polymer mobility on conductivity of single-ion conductors Kokonad Sinha, Janna Maranas Scientists are turning to the use of polymers as substitutes for liquid electrolytes in lithium ion batteries, because of their mechanical flexibility and non-toxic properties. Physical mixtures of lithium salt and poly(ethylene oxide) (PEO + LiClO4) are commonly chosen because they have potential for high ionic conductivities. However, high mobility of ions in these mixtures results in electrode polarization, which affects battery performance. To isolate the effect of the cation and to reduce the obstacle of concentration polarization, the anion is chemically incorporated into the backbone of the polymer, thereby rendering it immobile. These single-ion conductors are called ionomers. Neutron scattering experiments have been conducted on ionomers to observe the relation between ionomer mobility and ionic conductivity. Results show that with increasing ion content, there arises a new process at smaller length scales. Comparisons with PEO + LiClO4 systems hint at the formation of cation-PEO-anion complexes which are significantly slower in dynamics than the segmental motion of the polymer. This interaction between the cation and the polymer chain is of vital importance in understanding the fundamental mechanism of ion conduction in polymers. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D20.00011: Electrochemical Characterization of poly (styrene-b-ethylene oxide)/LiTFSI Lamellar Diblock Copolymer Electrolyte System Nitash Balsara, Ashoutosh Panday, Scott Mullin, Nisita Wanakule We present the electrochemical characterization studies of symmetric poly (styrene-b-ethylene oxide) copolymers (SEO) and Li[N(SO2CF3)2] (LiTFSI). The molar ratio of Li to ethylene monomers, r, was varied from 0.02 to 0.10. The ionic conductivity of these electrolytes increases with molecular weight over the entire range of temperatures and r values examined. Preliminary data suggest that the salt diffusion coefficient also increases with increasing MW of PEO block. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D20.00012: Progress toward few-molecule photochemistry with a low temperature STM. David Daughton, Donghun Lee, Jay Gupta Photochemistry at interfaces provides insight into molecular binding and charge transfer with future implications for organic photo-active devices. We have developed a novel instrument combining a low-temperature scanning tunneling microscope (STM) with a maneuverable, high numeric aperture lens in proximity to the tunnel junction for the study of photoactive systems with single molecule sensitivity. We will present the results of our initial efforts on the electronic and photo-induced polymerization of C60 islands, tunneling and photo-induced isomerization of thioindigo, and the effects of well-defined optical nanostructures on photochemical processes. http://www.physics.ohio-state.edu/$\sim $jgupta [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D20.00013: Confinement-Induced Fast Discharge and Low Dielectric Losses in Ferroelectric PVDF Graft Copolymers Lei Zhu, Fangxiao Guan, Zhongzhe Yuan The relatively high dielectric loss of poly(vinylidene fluoride) (PVDF) and its copolymers limits their range of application as a high energy density capacitor material, although a high electric energy density was recently reported for millisecond discharge. In this work, we report time independent (or fast) discharge and reduced losses in ferroelectric poly(vinylidene fluoride) (PVDF) graft copolymer dielectric films. Experimental results suggested that the fast discharge and low losses were results of an increased amorphous content and nanoscale confinement of ferroelectric PVDF crystals. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D20.00014: ABSTRACT WITHDRAWN |
Session D21: Semiconductors: Transport
Sponsoring Units: FIAPChair: Sergey Vitkalov, City University of New York
Room: 323
Monday, March 16, 2009 2:30PM - 2:42PM |
D21.00001: Unified Theory of Charge Transport in Wide-Band and Narrow-Band Semiconductors Frank Ortmann, Friedhelm Bechstedt, Karsten Hannewald The charge carrier mobility is often calculated within one of the two limiting cases: wide bands or narrow bands. In the case of wide-band systems, usually pure band transport is assumed along with a calculated relaxation time. In contrast, for narrow-band materials, hopping is usually considered prevalent and the interaction with lattice vibrations is described within the polaron concept. In this talk, we will present a unified approach to the description of charge transport based upon the Kubo formalism applied to a Holstein Hamiltonian. As a result, we obtain an analytical formula for the temperature dependence and anisotropy of the mobility describing a seamless transition from band transport at low temperatures to hopping transport at high temperatures. The results are illustrated for naphthalene crystals and a comparison to previous approaches [1,2] is made. \\[3pt] [1] V.M. Kenkre, Phys. Lett. A 305, 443 (2002)\\[0pt] [2] K. Hannewald and P.A. Bobbert, Phys. Rev. B 69, 075212 (2004) [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D21.00002: Carrier transport in nanodevices : a competitive playground for the Boltzmann and the Wigner distribution functions? Fons Brosens, Wim Magnus In principle, transport of charged carriers in nanometer sized solid-state devices can be fully characterized once the non- equilibrium distribution function describing the carrier ensemble is known. In this light, we have revisited the Boltzmann and the Wigner distribution functions and the framework in which they emerge from the classical respectively quantum mechanical Liouville equation. We have assessed the method of the characteristic curves as a potential workhorse to solve the time dependent Boltzmann equation for carriers propagating through spatially non-uniform systems, such as nanodevices. In order to validate the proposed solution strategy, we numerically solve the Boltzmann equation for a one- dimensional conductor mimicking the basic features of a biased low-dimensional transistor operating in the on-state. Finally, we propose a computational scheme capable of extending the benefits of the above mentioned solution strategy when it comes to solve the Wigner-Liouville equation. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D21.00003: Measurement and analysis of extraordinary electroconductance in Ti-GaAs hybrid structures S.A. Solin, A.K.M. Newaz, Y. Wang, J. Wu, W.-J. Chang, V.R. Kavasseri, I.S. Ahmad, I. Adesida, R. Bashir We present a comprehensive study of a new phenomenon, extraordinary electroconductance (EEC), in microscopic and mesoscopic metal-semiconductor hybrid structures (MSH) at room temperature with different geometrical characteristics. Our artificially designed MSH structures show highly efficient external electric field sensing properties not exhibited by bare semiconductor structures. The EEC device has been fabricated from a GaAs epitaxial layer with a Ti/Au shunt. When subject to an external electric field it gives a maximum 5.2$\%$ EEC effect corresponding to an external electric field resolution of 3.05V/cm at a bias field of 2.5 KV/cm. Moreover, the study reveals a strong dependence of the transport properties on the geometry of the MSH. An analytical 2-layer model is developed which provides good agreement with the experimentally observed data.\footnote{Y. Wang, {\textit{et. al.}}, {\bf{App. Phys. Lett.}}, 92, 262106 (2008)} We propose that scaled down nanoscopic EEC sensor arrays can be used as a novel technique for imaging the charge distribution on a single cell surface in real time. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D21.00004: Universal oscillations in counting statistics Christian Flindt, Christian Fricke, Frank Hohls, Tomas Novotny, Karel Netocny, Tobias Brandes, Rolf. J. Haug Noise and fluctuations are results of stochastic processes that originate from quantum or classical sources. Higher-order cumulants of the probability distribution underlying the stochastic events are believed to contain detailed information about the stochastic process, but they are often difficult to measure. In this talk we report the first measurements of the transient cumulants of the number of electrons passing through a quantum dot to very high orders (up to order 15) [1]. The cumulants grow factorially in magnitude with the cumulant order and show surprising oscillations as functions of measurement time. Based on theory for high-order derivatives in the complex plane we show that the oscillations in fact constitute a universal phenomenon, appearing as a function of almost any system parameter for a large class of stochastic systems. Our theory provides a unified interpretation of previous theoretical studies of high-order cumulants [2] as well as our new experimental data. [1] C. Flindt, C. Fricke, F. Hohls, T. Novotny, K. Netocny, T. Brandes {\&} R. J. Haug, submitted (2008). [2] C. Flindt, T. Novotny, A. Braggio, M. Sassetti {\&} A.-P. Jauho, \textit{Phys. Rev. Lett. }\textbf{100}, 150601 (2008). [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D21.00005: Bloch Oscillations of Electrons in a Quantum-Dot Superlattice Danhong Huang, S.K. Lyo Numerical results for both the transient and steady-state currents in a strong DC electric field are presented for electrons in a quantum-dot superlattice. A microscopic scattering model is applied to study the dynamics of electrons scattered by impurities and phonons based on the Boltzmann equation. Good agreement is found between the numerical results and a recent analytic solution under a relaxation-time approximation for electron-phonon scattering [S. K. Lyo, Phys. Rev. B 77, 195306 (2008)]. Different roles played by elastic and inelastic scattering on the damped Bloch oscillations and the nonlinear steady-state are demonstrated from our numerical results. We will also briefly discuss suppression of the dynamical localization by strong Bloch oscillations under an additional nonlinear AC field and opposite roles played by elastic and inelastic scattering on the damped dynamical localization. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D21.00006: Full Counting Statistics of a Quantum Point Contact with Time-dependent Transparency Jin Zhang, Yury Sherkunov, Nicholas d'Ambrumenil, Boris Muzykantskii Controlled injection of single electrons into a ballistic conductor is essential for ``moving-electron-based'' quantum computation. We consider an electron system in two 1D ballistic conductors separated by a tunneling barrier at zero temperature. We present numerical results for the full counting statistics (FCS) for the case when the barrier potential is modulated in the presence of a time-dependent bias voltage applied between the leads. The calculation is based on Abanov and Ivanov's formula[1]. For the case of a periodic input with a large number of cycles, we perform the calculation in discretized energy space and obtain the characteristic function $\chi(\lambda)$ as well as physical observables such as the current, noise and entanglement entropy directly. We show how to optimize the gate potential of a quantum point contact to generate a single electron excitation with minimal noise. [1]A.G.~Abanov and D.A.~Ivanov, {\em Phys.Rev.Lett.}, 100, 086602 [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D21.00007: Optimal electron entangler at low temperatures Yury Sherkunov, Jin Zhang, Nicholas d'Ambrumenil, Boris Muzykantskii Electron transport in mesoscopic contacts at low temperatures is accompanied by logarithmically divergent equilibrium noise when a tunneling barrier transparency is changed abruptly. We show that the equilibrium noise can be dramatically suppressed in the case of tunneling junction with smoothly tunable transparency. We study analytically the full counting statistics of the transparency modulated quantum contact and identify the minimal excitation states generated by the gate voltage. The proposed scheme could be used as an optimal electron entangler at low temperatures. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D21.00008: Observation of quantum phase shift in an Aharonov-Bohm ring with a fully controlled flying charge qubit Michihisa Yamamoto, Christopher Bauerle, Seigo Tarucha Aharonov-Bohm effect is one of the most typical interference phenomena of electrons. Although a number of experiments have been performed to date, observation of phase shift at each path has been rather difficult due to the phase rigidity in the two-terminal setup. In this study, we employed a hybrid device consisting of a parallel coupled-wire and an AB ring, in which each coherently propagating electron acts as a flying qubit. In this device, phase rigidity no longer exists as there are two output contacts. The qubit is defined as superposition of two quantum states: an electron exists in one of the two wires. Then, the inter-wire tunnel coupling gives flipping between the two quantum states, and the evolution of the phase in the AB ring is translated into rotation about the z-axis of the Bloch sphere. In the experiment, we defined the initial qubit state by injecting electrons into only one of the two wires, and obtained the arbitrary output state by tuning gate voltages. The output state also oscillates as a function of perpendicular magnetic field $B$ with the AB oscillation period. We observed the shift of k-vector in one of the two wires works equivalently as the shift of$ B$. This is the direct observation of the phase shift \textit{$\Delta \theta =\Delta $k$\cdot $L}. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D21.00009: Current dependent dephasing in an Aharonov-Bohm Interferometer Kuan-Ting Lin, Yiping Lin, J. C. Chen, T. Ueda, S. Komiyama We have studied the temperature dependence of the current induced dephasing rate in a ballistic GaAs/Al$_{x}$Ga$_{1-x}$As ring. The dephasing rate is linearly proportional to the temperature regardless of the current applied. The AB oscillations are suppressed by the increase of the excitation current; however, the dephasing becomes less temperature dependent. Our observations cannot be interpreted by Joule heating effect. Possible decoherence mechanisms caused by the excess current will be discussed. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D21.00010: Tunable Channel Interference in an Aharonov-Bohm Ring Yiping Lin, Pei-Jung Wu, Kuan-Ting Lin, J. C. Chen, T. Ueda, S. Komiyama We have investigated the Aharonov-Bohm effect in a quasi one-dimensional ring on a GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure, which is defined by two metallic arc gates coupled to each branch of the ring. Each gate can be separately biased to uniformly squeeze the channel width of electrons, thereby externally tuning the transverse modes in the interference paths. The oscillatory magnetoconductance of the device is systematically studied by varying the number of channels in each path. We have observed the evidence of phase shifts in the magnetoconductance oscillations due to the suppression of the mode numbers on the ring path. Though the periodicity is not well resolved, qualitatively our data support the random phase shifts between the successive modes. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D21.00011: Transport and noise in 90nm n-GaAs Epilayers A. Gilbertson, J.D. Moore, G. Perkins, J. Gallop, L.F. Cohen, A.K.M. Newaz, S.A. Solin Extraordinary Magnetoresistance (EMR) belongs to the family of
EXX effects which form the basis for a number of devices that
offer the potential for high sensitivity applications. Such
devices would benefit from minimising the active volume of the
sensor. To reduce that volume and minimize wafer fabrication
complexity it is desirable to employ unltra-thin GaAs epilayers.
Accordingly, we report here the transport and noise properties
of 90nm Si-doped GaAs films grown by molecular beam epitaxy which
have been fabricated into both microscopic EMR devices and
macroscopic van der Pauw geometries. These films exhibit a room
temperature electron mobility and density of 3225
$cm^2V^{-1}s^{-1}$ and1.45x$10^{17}cm^{-3}$, respectively, and
show only a 6\% variation over the temperature range $2K |
Monday, March 16, 2009 4:42PM - 4:54PM |
D21.00012: New Measurement of Diffusion Thermopower of 2D Electron Systems W.E. Chickering, J.P. Eisenstein, J.L. Reno The thermoelectric properties of low-dimensional electronic systems provide information about carrier transport that is complementary to that obtained from ordinary charge transport. In conventional measurements of the thermopower $S$ of two-dimensional electron systems (2DESs), phonon drag overwhelms the diffusion thermopower $S_d$ of the electron gas for temperatures $T > \sim 0.1$ K. We introduce a new hot electron thermocouple technique which vastly reduces the importance of phonon drag and allows us to accurately determine $S_d$ in a 2DES in a GaAs/AlGaAs heterostructure. Differentially gated 2DES channels provide the analogs of the dissimilar metals used in a conventional thermocouple. The device is calibrated via the temperature dependence of the longitudinal resistance of the 2DES at the thermocouple junction. Our results are in good quantitative agreement with the Mott formula for the temperature and density dependence of $S_d$ for temperatures up to $T \sim 2$ K. This work is supported by DOE grant DE-FG03-99ER45766 and Microsoft Project Q. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D21.00013: Thermopower of n- and p-type InN Nate Miller, Joel Ager, Rebecca Jones, Holland Smith, Kin Man Yu, Eugene Haller, Wladek Walukiewicz, William Schaff, Chad Gallinat, Gregor Koblmuller, Jim Speck The exceptionally large ($>$ 5.5 eV) electron affinity of InN leads to unique electronic properties such as surface electron accumulation and an extreme propensity for n-type conduction. This, combined with a small energy gap and strongly energy dependent effective mass, makes an analysis of charge transport and determination of band structure parameters an arduous task. In this work we show that thermopower (Seebeck coefficient) measurements can address some of the issues by providing a new tool to study the unique charge transport properties of InN and In-rich group III-nitride alloys. Our thermopower experiments are used to demonstrate the presence of mobile holes in Mg-doped InN providing the first direct, quantitative measurement of hole transport in InN. We also report modeling of the thermopower of n-type InN considering the various scattering mechanisms. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D21.00014: Measurement of [N] and T-dependence of electron effective mass in GaAsN Tassilo Dannecker, Yu Jin, John Buckeridge, Ctirad Uher, Cagliyan Kurdak, Stephen Fahy, Rachel S. Goldman The electron effective mass of GaAs$_{1-x}$N$_{x}$ is predicted to be dependent on N-composition, x, and temperature, T; however, conflicting results have been observed using cyclotron resonance and thermomagnetic measurements. Using thermopower and Hall measurements, in conjunction with assumptions of parabolic bands and Fermi-Dirac statistics, we determined the T-dependence of the electron effective mass of GaAs$_{1-x}$N$_{x}$, in comparison with that of GaAs. Measurements of the T-dependent Seebeck coefficient, S, for N compositions ranging from x=0 to 0.0100, reveal a decrease in dS/dT with increasing x. For GaAs, the free carrier concentration, [n], is independent of T. In all other cases, [n] increases (decreases) with T (x). For GaAs, the effective mass decreases from 0.06m$_{0}$ at 140K to 0.052m$_{0}$ at 300K, similar to literature reports.$^{4}$ For GaAsN, the effective mass apparently increases (decreases) with x (T), ranging from 0.1m$_{0}$ to 0.16m$_{0}$ at 140K, with values 14{\%} (40{\%}) lower for x=0.0075 (x=0.0100) at 300K. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D21.00015: Temperature dependence of electron mobilities in InN Leonardo Hsu, Wladek Walukiewicz InN allows the possibility of engineering nitride materials with bandgaps as small as 0.7 eV. We have calculated electron mobilities in InN taking into account the non-parabolicity of the conduction band, as well as the standard scattering mechanisms of acoustic and optical phonons and Coulomb scattering from charged impurities. Although our calculations explain well the experimentally measured mobilities at temperatures higher than about 200 K, the measured mobilities in lightly doped InN at low temperatures decrease in a way that cannot be accounted for by the standard theory. We discuss the characteristics and possible origins of the additional mechanism that must be included in the calculations in order to fit the experimental results. [Preview Abstract] |
Session D22: Focus Session: Spins in Group IV Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Bruce Kane, LPS, University of Maryland
Room: 324
Monday, March 16, 2009 2:30PM - 3:06PM |
D22.00001: Quantum control of donor electron charge and spin in Si close to a Si-SiO$_2$ interface Invited Speaker: Doped Si is a promising candidate for quantum information processing due to its potential for scalability, long spin coherence times, and the continuing progress on Si material processing, technology and miniaturization over several decades. I will discuss important issues for single- and two- qubit operations in Si-based quantum computer proposals involving P donors close to a SiO$_2$ interface. For a single donor, donor-bound electron manipulation between the donor and the interface by electric and magnetic fields is investigated [1,2]. Valley interference and how it affects a donor electron close to an interface under an applied electric field is also considered, taking the valley-orbit coupling at the interface as a parameter. It will be shown that, for nonzero interface valley-orbit coupling, this configuration leads to oscillatory behavior of the donor ionization time as a function of the donor-interface distance while the characteristic ionization field does not oscillate with distance [3]. The physical origin of these effects, and their impact in proposed operations of donor-based qubits, will be discussed. For a donor pair, the exchange coupling of interface electrons bound to the donors double well potential is calculated within the Heitler London approach [2,4]. The feasibility and convenience of performing exchange operations for electron pairs at the interface as opposed to around the donors will be assessed. Work done in collaboration with M.J.Calderon and S. Das Sarma and partially supported by LPS-NSA and MICINN-Spain. \\[4pt] [1] M.J. Calderon, B. Koiller, X. Hu, and S. Das Sarma, Phys. Rev. Lett. 96, 096802 (2006).\\[0pt] [2] M.J. Calderon, B. Koiller and S. Das Sarma, Phys. Rev. B 75, 125311 (2007).\\[0pt] [3] M.J. Calderon, B. Koiller and S. Das Sarma, Phys. Rev. B 77, 155302 (2008).\\[0pt] [4] A. L. Saraiva, M. J. Calderon, and B. Koiller Phys. Rev. B 76, 233302 (2007). [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D22.00002: A new electrical readout mechanism for Si:P qubits Dane R. McCamey, G.W. Morley, S.-Y. Paik, S.-Y. Lee, L.-C. Brunel, J. van Tol, C. Boehme Phosphorus donor spins in silicon are a promising candidate for the implementation of quantum bits, and electrical detection is viewed as the most promising route towards the single donor readout required to further advance such concepts. We will discuss a major limitation to commonly used electrical detection schemes. The standard way to electrically detect Si:P spin states involves utilizing spin dependent recombination with nearby probe spins, usually of defects at the Si-SiO$_2$ interface. This process has a fast, fixed timescale, thereby limiting coherence times. We find that these times are of order $1 \mu$s, in agreement with other studies. By moving to high magnetic fields ($B > 8$ T) we enter a new regime - complete electron polarization. This allows us to utilize a different readout mechanism, namely, capture into the donor D$^-$ state which causes a decrease in the photocurrent in the sample. We have developed a system which allows us to investigate the donor spin phase coherence times at these high magnetic fields; we find them to be over $100 \mu$s [1]. Additionally, the signal observed at these high fields is significantly larger ($\Delta I/I \sim 5$ \%) than at low fields, providing a pathway towards single spin detection. [1] PRL \textbf{101}, 207602 (2008) [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D22.00003: SU(4) Kondo in a single donor transport in a Si FinFET G.P. Lansbergen, G.C. Tettamanzi, A. Verduijn, M. Blaauboer, S. Rogge Recently, single dopants became experimentally accessible and there is a large effort to exploit their atomic characteristics in nano devices. Orbital Kondo effects in Si and SiGe are of fundamental interest since they explore the role of the valley degree of freedom in this material system. It has been theoretically predicted that the valley degeneracy leads to SU(4)-correlations which entangles the spin and momentum of exchanged electrons. Here, we experimentally study Kondo effects in a novel system, a single shallow donor in a three-terminal geometry. We use Si wrap-around gate (FinFET) devices with a single Arsenic donor atom in the channel dominating the sub-threshold transport characteristics. The ground state of this system originates from the hybridization of the donor hydrogen-like state which has no valley degeneracy (due to the strong valley-orbit interaction) and a quantum dot-like state which is two-fold valley degenerate. In the Coulomb-blockade regime with a single electron on the system we observe a set of transport resonances. We show these resonances to originate from Valley Kondo effects by means of their dependence on temperature, magnetic field, orbital splitting and their substructure. The entanglement between spin and momentum provides new opportunities for spin control in silicon. For example, we show that this device operates as a gated spin filter in Si with a potentially high transitivity. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D22.00004: Electric field control of spins in a silicon two-dimensional electron gas. R. Jansen, B.C. Min, S. P. Dash, R. S. Patel, M. P. de Jong A key objective in the development of semiconductor spintronics is the active control of spins in semiconductors. The manipulation by electric rather than magnetic fields is preferred as this is more efficient for nanoscale high frequency devices. Proposals for electric spin control, for example for use in a spin transistor, have so far focused on mechanisms that require spin-orbit interaction. Unfortunately, in silicon, the mainstream semiconductor, the weak spin-orbit interaction renders these mechanisms unsuited. Hence, alternative approaches are paramount to the success of semiconductor spintronics. Here we demonstrate spin control by electric fields in a silicon two-dimensional electron gas (2DEG), exploiting the discrete electronic structure of the 2DEG. This, in combination with an electric field, allows spin manipulation without the need for spin-orbit interaction. The spin control is manifested as resonances in the tunnel magnetoresistance between the Si 2DEG and a ferromagnetic tunnel contact, with amplitude of up to 8{\%}. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D22.00005: Injection and extraction of spins in a Silicon lateral transport structure Olaf van 't Erve, Chaffra Awo-Affouda, Aubrey Hanbicki, Michael Holub, Connie Li, Phillip Thompson, Berend Jonker Significant progress has recently been made on spin injection into the technologically important semiconductor, Si. A nonlocal measurement technique, which excludes spurious contributions from AMR and local Hall effects, was used to show lateral diffusive spin transport through silicon using Fe/Al2O3 surface contacts. The tunnel contacts are used to create and analyze the flow of pure spin current in a silicon transport channel. The nonlocal signal shows that a spin current can be electrically detected after diffusive transport through the silicon transport channel and the signal depends on the relative orientation of the magnetization of the injecting and detecting contacts. Hanle effect measurements up to 125 K demonstrate that the spin current can be modulated by a perpendicular magnetic field, which causes the electron spin to precess and dephase in the channel during transport. By changing the bias on the injector contact we can either inject or extract spin from the Silicon channel. Here we will show using Hanle and lateral spin- valve measurements that we can change the polarization of the spin accumulation by going from the injection regime to the extraction regime and we will compare the efficiency of spin- injection versus spin extraction. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D22.00006: Spin Precession in Oblique Magnetic Fields Jing Li, Biqin Huang, Ian Appelbaum Spin precession and dephasing (``Hanle effect'') provide an unambiguous means to establish the presence of spin transport in semiconductors. We compare theoretical modeling with experimental data from drift-dominated silicon spin-transport devices, illustrating the non-trivial consequences of employing oblique magnetic fields (due to misalignment or intentional, fixed in-plane field components) to measure the effects of spin precession. Model results are also calculated for Hanle measurements under conditions of diffusion-dominated transport, revealing an expected Hanle peak-widening effect induced by the presence of fixed in-plane magnetic bias fields. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D22.00007: Geometric dephasing-limited Hanle effect in long-distance lateral silicon spin transport devices Biqin Huang, Hyuk-Jae Jang, Ian Appelbaum Using ballistic injection and hot-electron spin filter detection, lateral spin transport over 2 millimeters is demonstrated in undoped single-crystal Silicon. In these devices, geometrically-induced dephasing (Hanle effect) is so strong that the effects of spin precession could not be measured with only a single-axis magnetic field. However, a two-axis magnetic field can be used to obtain unequivocal evidence of spin precession and transport despite full dephasing. We therefore conclude that there is never a reason to avoid measurement of spin precession as unequivocal evidence of spin transport in semiconductor devices. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D22.00008: Non-ohmic spin transport in n-type doped silicon H.-Jae Jang, Jing Xu, Jing Li, Biqin Huang, Ian Appelbaum In contrast with undoped silicon transport layers [1], conduction-band bending in n-type doped silicon spintronic devices results in non-ohmic spin-polarized electron transport [2]: for low applied voltage drops across the transport layer, a potential well causes confinement of electrons in the silicon transport layer, and they must diffuse against an electric field to escape. Numerical simulation using a Monte Carlo algorithm reveals that the average transit time across our 3.3 um Si layer can be changed over 4 orders of magnitude by varying an applied voltage. We can therefore deduce a long spin lifetime [3] in n-type doped silicon from comparison between experimental data and fitting-parameter-free simulation results in spite of the short transport distance. References [1] Ian Appelbaum et al. Nature 447, 295 (2007). [2] H.-Jae. Jang et al. Phys. Rev. B 78, 165329 (2008). [3] Biqin Huang et al. Phys. Rev. Lett. 99, 177209 (2007). [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D22.00009: Indirect Optical Injection of Carriers and Spin in Silicon JinLuo Cheng, Julien Rioux, John Sipe Degenerate two-photon indirect absorption in silicon is an important limiting effect on the use of silicon structures for all-optical information processing at telecommunication wavelengths. Optical injection of spins in silicon is potentially important for spintronics applications. We theoretically investigate one- and two-photon indirect absorption in silicon, using a pseudopotential description of energy band and the adiabatic bond charge model to describe phonon dispersion and polarization. Spin injection is calculated as well. We compare our results with experiments. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D22.00010: Transient Current Spectroscopy of a Si Quantum Dot Ming Xiao, Hongwen Jiang We present a transient current spectroscopy study of a Si-MOS based quantum dot. The study was conducted in the few electron region. A voltage pulse pumped the electrons into an excited orbital state and the non-equilibrium transient current through the dot was recorded. The evolution of the excited state as a function of magnetic field shows signatures of a transition from a spin singlet state to a triplet state of an electron pair. A pump-and-probe technique was employed to set a lower limit of the triplet-singlet relaxation time. The work was sponsored by United States Department of Defense. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D22.00011: Magneto-transport properties of Si-based Nanowires Sungmu Kang, Jugdersuren Battogtokh, Andrew C. Buchele, David A. McKweon, Ian L. Pegg, John Philip We report the growth and magneto-transport properties of Si-based, Mn$_{5}$SiC nanowires grown using chemical vapor deposition. High resolution transmission electron microscopy and x-ray diffraction studies show that the nanowires crystallize in Mn$_{5}$SiC orthorhombic structure. Ferromagnetic Mn$_{5}$SiC nanowires were grown using a coordination complex-based precursor. In the presence of an external magnetic field, Mn$_{5}$SiC nanowire-based devices exhibit spin dependent transport properties at room temperature. A large change in current with almost two orders of magnitude increase is observed when a small field is applied parallel to the axial direction of the nanowire. We will discuss in details the magneto-transport properties of Mn$_{5}$SiC nanowire based devices. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D22.00012: Prospects of Spin Injection in Germanium Nanowires En-Shao Liu, Kamran Varahramyan, Junghyo Nah, Sanjay Banerjee, Emanuel Tutuc Efficient spin injection from ferromagnetic (FM) contacts into semiconductors (SC), the prerequisite for spin-based semiconductor devices, is typically suppressed by the conductivity mismatch between the FM contact and the SC. A significant spin injection can be achieved however if the contact resistivity at the FM/SC interface is appropriately engineered [1]. We report here contact resistivity measurements of n-type germanium (Ge) nanowires (NWs) with two FM metals, namely permalloy (Ni$_{80}$Fe$_{20})$ and nickel (Ni), for NW doping densities between 10$^{16}$ and 10$^{20 }$cm$^{-3}$, and for temperatures between 77K to 300K. Using back-gated two- and four-terminal measurements, we show that the contact resistivity varies from 10$^{-7 }(\Omega $ cm$^{2})$ for highly-doped NWs, to 10$^{-4 }(\Omega $ cm$^{2})$ for moderately-doped NWs. Within the framework of the spin injection theory [1], these values indicate that by optimizing the device parameters, namely the choice of FM metal, contact width, NW diameter and doping density, spin injection into Ge NWs is possible. [1] A. Fert and H. Jaffres, Phys. Rev. B. \textbf{64}, 184420 (2001) [Preview Abstract] |
Session D23: Diffusion and Transport Properties
Sponsoring Units: DCMPChair: Duane Johnston, University of Illinois at Urbana-Champaign
Room: 325
Monday, March 16, 2009 2:30PM - 2:42PM |
D23.00001: A multiscale study of hydrogen embrittlement of metals: Revisiting the hydrogen enhanced local plasticity (HELP) mechanism Johann von Pezold, J\"{o}rg Neugebauer The embrittlement of metals by H is a long-standing problem, whose underlying mechanisms are still largely unclear. Here we consider the atomistic basis of the HELP mechanism, which asserts that H mobilises dislocations by shielding elastic dislocation-dislocation interactions. Using a combination of density-functional theory calculations, semiempirical EAM potentials and an effective lattice gas Hamiltonian we determine the interaction of H with the strain field around edge dislocations in Ni. Our results reveal an attractive, but short ranged interaction between H interstitials, which leads above a critical concentration to the formation of a precursor hydride phase. The increased lattice parameter of this hydride phase induces significant misfit stress, but localises the dislocation stress field due to a localisation of the dislocation strain field. For H concentrations of up to 0.25 at. \%, the stress localisation is found to dominate and thus effectively shields the stress field of the dislocation, pointing towards a novel atomistic basis for the HELP mechanism. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D23.00002: DFT energetics of hydrogen binding to point defects in iron and steels William Counts, Chris Wolverton, Ron Gibala It is well known that hydrogen degrades the properties of iron and steel. One proposed mechanism of hydrogen attack is the concept of ``hydrogen traps'', which are generally microstructural material defects that bind hydrogen atoms. The stability and energetics of a number of potential traps (like vacancies, interstitials, and substitutional alloying elements) in bcc and fcc iron are investigated using density functional theory (DFT). We find that hydrogen is very sensitive to its local environment. For example, inIn a perfect bcc iron lattice, hydrogen prefers to sit in the tetrahedral site rather than the octahedral site or a substitutional site, and there is a repulsive interaction between carbon and hydrogen. In the presence of a vacancy, the energy barrier between the octahedral and tetrahedral sites disappears and hydrogen only resides in the octahedral sites. The hydrogen-vacancy binding energy changes as more hydrogen atoms bind to the vacancy, but the binding energy does not significantly change in the presence of carbon. We also find that H$_{2}$ molecules are not stable inside a vacancy: they dissociate spontaneously (without a barrier) into octahedral positions. Details about vacancy and other potential traps are presented and their roles in hydrogen embrittlement are discussed. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D23.00003: First-principles studies of helium in palladium tritides: PdT$_{x}$ (0$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $x$\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle=}\vphantom{_x}}$}} $1) Pei Lin, Yan Wang, M.Y. Chou Helium bubbles have significant impact on the stability and mechanical properties of materials used in nuclear-energy systems. Theoretical studies on the behavior of He in various metals have been reported in the past. However, few studies have taken into account the effect of the coexistence of He and tritium ($^{3}$H) in the metal lattice. We have performed first-principles calculations of He inside palladium tritides with various H concentrations. Instead of interacting with He impurities directly, the interstitial H mainly modifies the electronic structure of the metal lattice. The Pd d-orbital near the Fermi-level shifts downward as H atoms occupy the interstitial sites, which in turn promotes the interaction between He 2-s and Pd 4-d states. We have examined the changes in the formation energy of a He impurity with various H concentrations. The effect will further modify the energetics of trapping multiple He atoms to form a cluster inside the hydrogenated Pd system. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D23.00004: Calculating self diffusion in Mo using the AM05 density functional Thomas R. Mattsson, Nils Sandberg, Rickard Armiento, Ann E. Mattsson Vacancy diffusion is a major mechanism of mass transport in solids. While the motion of vacancies and interstitials is largely understood for fcc metals like aluminum [1], important questions remain for bcc metals. We present first principles and model potential simulations of self-diffusion in Mo, compare the results to available experimental data, and discuss the magnitude and origin of different contributions to the diffusion. The density functional AM05 [2] is employed to calculate formation energies and diffusion barrier for vacancy migration. AM05 has been successfully applied to a wide range of different solids [3] and is shown to perform well also for Mo. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] N. Sandberg, B. Magyari-Kope, and T.R. Mattsson, PRL 89, 065901 (2002). [2] R. Armiento and A. E. Mattsson, PRB 72, 085108 (2005). [3] A.E. Mattsson, et al. JCP 128, 084714 (2008). [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D23.00005: Prediction and Modeling of Atomic Mobility in Alloys Zi-Kui Liu Atomic diffusion is a common and important non-equilibrium process in solids that takes place at finite temperatures. To computationally simulate atomic diffusion processes, the thermodynamic and atomic mobility databases of the materials of interest are needed. The modeling technique of atomic mobility databases and related software has been becoming more and more matured in the last decades. However, the input data for the modeling is exclusively taken from experimentally measured tracer and chemical diffusion coefficients. In this presentation, our recent progress in predicting self and dilute diffusion coefficients by quantum mechanics calculations will be discussed [1]. Our approach to the unstable vibrational mode of transition states during diffusion will be presented. The contribution to phenomenological modeling of atomic mobility will be briefed. \\[4pt] [1] M. Mantina, Y. Wang, R. Arroyave, L. Q. Chen, Z. K. Liu and C. Wolverton, ``First-principles calculation of self-diffusion coefficients,'' Phys. Rev. Lett., Vol.100, 2008, 215901. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D23.00006: Analysis of Cd jump rates among the two Ga sublatices in Ga$_7$Pd$_3$ using a stochastic model of hyperfine interactions M. O. Zacate, W. E. Evenson, G. S. Collins Atomic jump rates of Cd among the two inequivalent Ga sublattices in Ga$_7$Pd$_3$ were measured using perturbed angular correlation spectroscopy (PAC). Atomic jumps result in a reorientation and/or change in strength of electric field gradients experienced by the Cd PAC probes. Spectra were fitted to model functions generated using a stochastic model for fluctuating EFGs under the assumption that Cd probes can jump to nearest neighbor sites of each Ga sublattice. Jump activation energies were determined by fitting spectra collected at different temperatures simultaneously under the constraint that jump rates obey Arrhenius behavior. The following activation energies were determined for intra- sublattice jumps: 0.52(1) eV for Ga(3)$\rightarrow$Ga(3) and 0.25(4) eV for Ga(4)$\rightarrow$Ga(4) and for inter-sublattice jumps: 0.6(3) eV for Ga(3)$\rightarrow$Ga(4) and 0.47(9) eV for Ga(4)$\rightarrow$Ga(3), in which Ga(4) denotes the site with .3m point symmetry. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D23.00007: Vanadium diffusion in ferrite Sungho Kim, Jeffrey Houze, Seong-Gon Kim, Mark Horstemeyer Vanadium is a very important additive in high strength steel alloys. Small amount of vanadium addition strengthens the steel alloys. As a first step to study the effect of vanadium addition in steel alloys we calculated the diffusion path and the migration energy of vanadium atom in ferrite crystal using first principles calculations. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D23.00008: Anisotropic physical properties of RSn$_{2}$ (R = Gd - Lu) single crystals Emilia Morosan, Michael Mehlman, Liang Zhao, Thomas Sanders A wealth of magnetic and electronic properties (including metamagnetism, spin glass, non Fermi liquid behavior etc) has been observed in rare earth intermetallic compounds. The availability of these systems in single crystal form is imperious for characterizing their complex behavior. We are reporting the anisotropic physical properties of single crystals of the orthorhombic RSn$_{2}$ compounds (R = Gd - Lu). Complex anisotropic H -- T phase diagrams are observed in GdSn$_{2}$, which has an antiferromagnetic ground state below 27.5 K, with two more transitions at lower temperatures. The magnetic field induces metamagnetic phase transitions in several of the R members of the series. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D23.00009: Magnetostructural properties of RCo$_{2}$ (R = rare earth) compounds Durga Paudyal, Ya. Mudryk, V. K. Pecharsky, K. A. Gschneidner, Jr. First principles electronic structure calculations show that the cubic Laves phase (C15) is stable for GdCo$_{2}$ and TbCo$_{2}$, while the ground state structures of DyCo$_{2}$, HoCo$_{2}$, and ErCo$_{2}$ are tetragonal distortions of the cubic C15, orthorhombic, and rhombohedral structures, respectively. The R and Co moments exhibit antiparallel coupling, forming a ferrimagnetic state in every ground state structure of RCo$_{2}$. The predicted magnetostructural properties of RCo$_{2}$ are in good agreement with x-ray powder diffraction and magnetic measurements. The spin splitting of the conduction electrons due to the indirect R-R exchange plays a crucial role in the magnetostructural transformations in the R = Dy, Ho, and Er RCo$_{2}$ phases, and accounts for its absence in the monomorphic GdCo$_{2}$ and TbCo$_{2}$ compounds. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D23.00010: Transition in NiMnSn and NiMnIn C.P. Opeil, J.C. Lashley, J.L. Smith, T. Planes, L. Manosa Magneto-transport, specific heat, magneto-striction and temperature dependent UV photoemission are used to explore the martensite transition of the ferromagnetic shape memory alloys Ni$_{x}$Mn$_{y}$In$_{z}$ and Ni$_{x}$Mn$_{y}$Sn$_{z}$. Comparisons will be made to a previous work\footnote{Opeil, et al. \textit{Physical Review Letters} \textbf{100}, 165703 (2008).} on the stoichiometric single crystal Ni$_{2}$MnGa which reveals a temperature (235 \underline {$>$} T \underline {$>$} 190 K) and field dependent (0 -- 1 T) positive/negative magneto-resistance slope. Our experimental results will be discussed in light of a possible pseudo-gap formation coincident with the martensite transition in the two off-stoichiometric alloys. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D23.00011: Measurements of persistent currents in normal metal rings with cantilever torsional magnetometry William Shanks, Ania Bleszynski Jayich, Bruno Peaudecerf, Jack Harris We have measured the magnetization of arrays of micron-scale aluminum rings at low temperatures and in high magnetic fields using cantilever torsional magnetometry. We see clear evidence of normal state persistent currents in these rings. The current's dependence on magnetic field, temperature and the rings' circumference is consistent with theoretical predictions (due to Riedel and von Oppen) for non-interacting electrons in the diffusive regime. To fully characterize these samples, we also measured the magnetization of codeposited rings in the superconducting state and the magnetoresistance of codeposited wires. Together these measurements provide an especially clear picture of the normal-state persistent currents. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D23.00012: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:54PM - 5:06PM |
D23.00013: AC-condutance of a quantum chaotic cavity (semiclassical approach) C. Petitjean, D. Waltner, J. Kuipers, I. Adagideli, K.R. Richter Due to the progress made in the control and the manipulation of mesoscopic structures driven by high frequency periodic voltages, the ac regime has been recently experimentally investigated [1] and consequently its theoretical interest has been renewed. We consider here, a quantum chaotic cavity that is coupled via tunnel barriers and gates to a macroscopic circuit which contains ac-sources [2]. By extending to the ac-transport, the recent trajectory-based semiclassical theory of quantum chaotic transport in presence of tunnel barrier [3], we derive for arbitrary tunneling rates and arbitrary positive Ehrenfest time, the averaged and the weak-localization correction to the screened conductance. Then we use these results to investigate the effect of dephasing on the relaxation resistance of a chaotic capacitor in the linear low frequency regime. This last investigation are in principle relevant to the recent measure of the admittance at zero magnetic flux of a mesoscopic capacitor [1,4].\\ {\bf References}\\ \newcommand\itm[2]{\parbox[t]{1cm}{#1}\parbox[t]{14cm}{#2}} \itm{[1]} {J. Gabelli et al., Science {\bf 313}, 499 (2006).}\\ \itm{[2]} {C. Petitjean et al, {\it in preparation } (2008).}\\ \itm{[3]} {R.S.~Whitney, Phys. Rev. {\bf B, 75}, 235404 (2007).}\\ \itm{[4]} {S.~Nigg and M.~B\"uttiker, Phys. Rev. {\bf B 77}, 085312 (2008).} [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D23.00014: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 5:18PM - 5:30PM |
D23.00015: Evidence for Conditioning as a Progenitor of Double-C Transformation Mechanisms in Pu-Ga Alloys. J.R. Jeffries, K.J.M. Blobaum, M.A. Wall, A.J. Schwartz By alloying Pu with Ga, the fcc $\delta$ phase can be retained down to room temperature. This metastable $\delta $ phase is realized due to slow Ga diffusion, which prevents the $\delta$ phase from decomposing into the equilibrium mixed phase structures. The metastable $\delta$ phase in a Pu-1.9 at.{\%} Ga alloy, however, does yield to chemical driving forces by undergoing the $\delta \quad \to \quad \alpha $' isothermal martensitic transformation below M$_{s} \quad \approx $ -100 \r{ }C. This transformation exhibits poorly understood double-C behavior in the time-temperature-transformation diagram. Recently, a ``conditioning'' treatment---which entails isothermally holding a specimen at sub-anneal temperatures but above M$_{s}$ ---has been shown to dramatically affect the amount of $\alpha $' phase formed at low temperature. We report evidence that the conditioning treatment induces the lower-C of the double-C curve, and we implicate the classical nucleation of equilibrium phases as the underlying mechanism behind conditioning in Pu-Ga alloys. [Preview Abstract] |
Session D24: Focus Session: Nanotube manipulation and processing
Sponsoring Units: DMPChair: Michael Mehl, Naval Research Laboratory
Room: 326
Monday, March 16, 2009 2:30PM - 3:06PM |
D24.00001: Carbon nanotube devices: Sorting, Assembling, Characterizing Invited Speaker: Carbon nanotubes have been studied extensively over the last decade. Various exceptional properties have been revealed which still drive the vision about using carbon nanotube in future electronics, for instance as molecular nanoscale transistors or electromigration resistant interconnects. For many years a major obstacle was the inability to grow nanotubes with defined dimensions (length, diameter) and electronic properties (metallic,semiconducting). Recently those problems have been solved to a large extent by advanced sorting techniques. Today the challenge is to assemble nanotubes devices with defined properties to form a complex circuitry. As progress is made in making highly-integrated nanotube device arrays new characterization techniques have to be developed which allow testing large number of devices within an acceptable time. Along this line I will report on the state-of-the-art of sorting of carbon nanotube, as a base for nanotube device fabrication [1]. I will then explain our strategy to assemble high-density arrays of nanotube devices [2] and discuss a new characterization technique for nanotube devices [3]. Finally I will introduce a novel device engineering tool [4]. \\[4pt] [1] R. Krupke et al., ``Separation techniques for carbon nanotubes'' in Chemistry of Carbon Nanotubes, p.129-139, American Scientific Publishers 2008\\[0pt] [2] A. Vijayaraghavan et al., ``Ultra-Large-Scale Directed Assembly of Single-Walled Carbon Nanotube Devices'', Nano Lett. 7 (2007) 1556-1560\\[0pt] [3] A. Vijayaraghavan et al., ``Imaging Electronic Structure of Carbon Nanotubes by Voltage-Contrast Scanning Electron Microscopy'', Nano Resarch 1 (2008) 321-332\\[0pt] [4] C. W. Marquardt et al., ``Reversible metal-insulator transitions in metallic single-walled carbon nanotubes'', Nano Lett. 9 (2008) 2767-2772 [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D24.00002: Self-healing and adsorbate-induced removal of defects on graphene and carbon nanotubes Leonidas Tsetseris, Sokrates Pantelides The presence of point defects is known to induce significant changes in the electronic, chemical, transport, and mechanical properties of graphitic systems. Here, we use first-principles calculations based on density-functional theory to describe several adatom-related processes that alter key physical traits of graphene and carbon nanotubes. We find that, while pairs of C adatoms and clusters of four or more self-interstitials stay idle unless the system is heated to very high temperatures, clustering of three C adatoms leads to removal of hillock-like features and creates mobile species, resulting in self-healing of defective structures. We also demonstrate the reactivity of defect pairs using hydrogen and oxygen as prototype adsorbates, and we show that interaction with extrinsic species is an alternative healing mechanism for adatom structures in the above systems. The results relate to the evolution of defects either during growth of carbon nanotubes or during post-growth treatment and operation of related devices. This work was supported in part by DOE Grant DEFG0203ER46096. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D24.00003: Self-similar mechanics of vertically aligned carbon nanotube organization Michael De Volder, Sameh Tawfick, Daniel Vidaud, A. John Hart It is well-known that carbon nanotube (CNT) growth from a dense arrangement of catalyst nanoparticles creates a self-organized vertically aligned CNT ``forest'' that offers attractive anisotropic mechanical, thermal, and electrical properties. Self-organization is governed by the CNT diameter and spacing, and the surface interactions between contacting CNTs. We demonstrate that arrays of CNT microstructures having micron-scale diameter organize in a similar manner as individual CNTs within a forest. For example, as postulated for CNT forests, entanglement of CNT microstructures during the initial stage of growth creates a self-supporting network, and this enables coordinated subsequent growth of the structures in the vertical direction. The alignment of these self-similar CNT forests is inversely related to the spacing of the microstructures, and like individual CNTs, widely-spaced microstructures that are not self-supporting fail to organize into an oriented superstructure. The growth rate and final forest height also depend on these geometric conditions, suggesting that mechanical interactions affect the collective progression and termination of a CNT film. This study and method offers new insights into the self-organization of one-dimensional nanostructures, and coordinated assembly of CNT microstructures offers opportunity for engineering energy-absorbing foams and photonic crystals. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D24.00004: Debundle of Single-Walled Carbon Nanotubes with Exfoliated Nanoplatelets Dazhi Sun, William Everett, Chien-Chia Chu, Hung-Jue Sue We report a simple and effective colloidal method to disperse single-walled carbon nanotubes (SWNTs) down to individual-tube level by utilizing exfoliated nanoplatelets in various solutions and polymer matrices. This approach yields a substantial amount of individual tubes without compromising their physical properties. The de-bundling and dispersion of SWNTs are confirmed by high-resolution transmission electron microscopy, UV-vis-NIR and Raman spectroscopy. After incorporated into polymers, SWNTs maintain individual dispersion. The dispersion mechanisms and implications of this approach are also discussed. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D24.00005: Motion and Manipulation of Suspended Single-Walled Carbon Nanotubes in Solution Ya-Qiong Xu, Arthur Barnard, Paul McEuen We have developed an optoelectronic imaging system which combines nanotube transistors with optical trapping techniques and the scanning photocurrent microscopy to investigate the motion of suspended single-walled carbon nanotubes in solution. This setup enables us to study the movement of nanotubes by monitoring their photocurrent images and to measure their thermal fluctuations through observing the movement of microbeads that are tightly attached to nanotubes by single-stranded DNA. By analyzing their thermal fluctuations, we are able to obtain the torsional and transversal stiffness of nanotubes and then calculate their diameters. We can also manipulate their motions by using an optical trap to pull on microbeads attached to nanotubes. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D24.00006: Processing of SWNT Dispersions -- Microfluidic Processing vs. Ultrasonication Tao Liu, Sida Luo, Chuck Zhang, Ben Wang Ultrasonication is the most commonly used processing technique for dispersing SWNTs in various media. High-power sonication enables the desired dispersability enhancement and the exfoliation of large SWNT bundles to smaller and even individual tubes. However, one disadvantage for this process is that the SWNT particles can be cut to shorter length. Using a newly developed characterization technique by us for quantifying the structures of SWNTs in a dispersion, we investigated the structural changes of SWNTs due to two different dispersion processing techniques, namely, microfluidic processing and ultrasonication. As a result, the microfluidic processing method shows significantly improved exfoliation efficiency as compared to ultrasonication. Moreover, the length of the exfoliated SWNT particles is maintained upon microfluidic processing, in contrast to the cutting effect caused by the high power sonication. In this presentation, we will discuss in-depth on the processing-structure-property relationships of SWNT dispersions processed by these two different processing methods. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D24.00007: Steric Mode Separation of Nanotubes Using Electric Field, Field-Flow Fractionation Frederick Phelan, Barry Bauer A Brownian dynamics simulation is used to study the separation of rodlike particles in Electric Field, Field-Flow Fractionation (EF-FFF), in which in addition to the FFF cross-flow, a uniform AC field acts in the gradient direction. Under these conditions, the electric field acts to align the tubes in the gradient direction in competition with both the shear field and Brownian motion. The simulation results show that as the rods become increasingly aligned, they undergo a transition from normal mode to steric mode separation. By exploiting field conditions in which either metallic or semi-conducting types are preferentially oriented relative to the other, this can be used in the context of nanotube separation as a means for separating tubes by type. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D24.00008: Thermal conductivity of vertically-aligned single-walled carbon nanotube film measured by 3$\omega$ method Kei Ishikawa, Saburo Tanaka, Koji Miyazaki, Junichiro Shiomi, Shigeo Maruyama Single-walled carbon nanotubes (SWNTs) have been expected to have extremely high thermal conductivity. However, the previously reported modeling and experimental works using individual SWNTs are too idealistic for the vertically-aligned single-walled carbon nanotube (VA-SWNT) film, in terms of defects, bundling effects, etc. In this work, we measured thermal conductivity of high purity VA-SWNT film synthesized by alcohol catalytic chemical vapor deposition (ACCVD) method [1]. We utilize thin film 3$\omega$ method for measuring thermal properties by depositing metal directly onto the VA- SWNT film. In the course of probing the intrinsic thermal conductivity, we discuss the effect of thermal boundary resistances at the nanotube-metal and nanotube-susbstrate boundaries. [1] Y. Murakami et al., Chem. Phys. Lett., 385 (2004), 298. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D24.00009: Optimal matching of thermal vibrations into carbon nanotubes K.G.S.H Gunawardana, Kieran Mullen Carbon nanotubes (CNTs) are promising candidates to improve the thermal conductivity of nano-composites. The main obstacle to these applications is the extremely high thermal boundary (Kapitza) resistance between the CNTs and their matrix. In this work our goal is to maximize the heat flux through the CNT by functionalizing their ends. We develop theoretical continuum models in which we vary the elasticity and density from surrounding medium to the CNT so as to maximize the transmission of thermal vibrations. We calculate the transmission coefficients using a scalar wave equation. Since the transport in CNT is strictly one dimensional, a Landauer formula is used to estimate the heat flux into the CNT. We determine the optimal continuous variation of elasticity and density with position for different geometries. We also investigate how to optimally match the nano-tubes to their matrix using a small number of discrete interfaces. Finally, we discuss the implications of these models for experiment. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D24.00010: Temperature dependence of the anharmonic decay of optical phonons in carbon nanotubes and graphite Ioannis Chatzakis, Hugen Yan, Daohua Song, Stephane Berciaud, Tony F. Heinz We report on the temperature dependence of the anharmonic decay rate of zone-center optical phonons in both single-walled carbon nanotubes and graphite from cryogenic temperatures to 650K. The measurements are performed using a pump-probe Raman scattering scheme with femtosecond (fs) laser pulses [Song et al. PRL 100,225503(2008)]. A nonequilibrium population of the zone-center (G-mode) optical phonons is created by an initial fs laser pulse. A subsequent fs probe pulse generates both Stokes and antiStokes Raman scattering, from which we infer the mode population of the G-mode phonons. We observe a large nonequilibrium phonon population in both systems, together with a room-temperature population lifetime of 1-2ps. The population decay is attributed to anharmonic coupling to lower-energy phonons [Bonini et al. PRL 99,176802(2007)]. We observe little T dependence of the decay rate below room temperature, but find a component growing roughly linearly with increasing T for $>$300K. We compare the behavior observed in nanotubes and graphite and discuss the implications of our results for the mechanism of the anharmonic decay of optical phonons in both systems. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D24.00011: Universal response of single-wall carbon nanotubes to radial compression: theory and experiment Helio Chacham, Ana Paula M. Barboza, Bernardo R. A. Neves Since the early 90's, the electronic and structural properties of single-wall carbon nanotubes (SWNTs) have been thoroughly investigated. Regarding SWNT mechanical properties, most of the attention has been given to their large resistance to axial tension, even though several electromechanical effects have been observed on radially compressed SWNTs, such as the predicted [1], and recently observed [2], metal-insulator transition. The present work brings a unifying picture to the process of radial compression/deformation of SWNTs, where experimental data are analyzed through a rescaling model yielding a universal-type behavior. Specifically, our AFM measurements show that the quantity $Fd^{3/2}(2R)^{-1/2}$, where $F$ is the force applied by the AFM tip (with radius $R$) and $d$ is the SWNT diameter, is a universal function of the compressive strain. Such universality is reproduced analytically in a model where the graphene bending modulus is the only fitting parameter. The application of the same model to the radial Young modulus $E_r$ leads to a further universal-type behavior that explains the large variations of the SWNTs $E_r$ reported in the literature. [1] M. S. C. Mazzoni and H. Chacham, Appl. Phys. Lett. 76, 1561 (2000). [2] A. P. M. Barboza et al., Phys. Rev. Lett. 100, 256804 (2008). [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D24.00012: Carbon Nanomaterials Under Highly Energetic Heavy Ion Irradiation J.M. Callahan, B.W. Jacobs, K. McElroy, M.A. Crimp, R.M. Ronningen, A.F. Zeller, H.C. Shaw The radiation performance of carbon nanomaterials: carbon onions and single-walled carbon nanotubes under highly energetic heavy ion irradiation was investigated, with highly oriented pyrolytic graphite (HOPG) used as the control. Samples were irradiated with a krypton-86 beam at 142 MeV/nucleon, a krypton-78 beam at 140 MeV/nucleon, and a calcium-48 beam at 140 MeV/nucleon and 70 MeV/nucleon at the National Superconducting Cyclotron Laboratory at Michigan State University. Fundamental structural and chemical modifications were investigated using Micro Raman spectroscopy and high-resolution transmission electron microscopy (HRTEM). Results indicated that the radiation resiliency of the single-walled carbon nanotubes exceeded that of highly oriented pyrolytic graphite, while the carbon onions showed structural modifications of the outer onion layers in the form of faceting and onion fusion. [Preview Abstract] |
Session D25: Focus Session: Graphene III: Growth and Structure
Sponsoring Units: DMPChair: Alessandra Lanzara, University of California, Berkeley
Room: 327
Monday, March 16, 2009 2:30PM - 2:42PM |
D25.00001: Synthesis, characterization and thermal stability of CVD-grown graphene nanoribbons Jessica Campos-Delgado, Y.A. Kim, Endo Morinobu, Katsumi Kaneko, Humberto Terrones, Mildred S. Dresselhaus, Mauricio Terrones A route to produce large amounts of nanometer scale graphene ribbons is presented. The process involves the thermal decomposition of ethanol-ferrocene and minute concentrations of thiophene solutions. The material consists of stacked graphene sheets with dimensions of several microns in length, 100-500 nm in width and 10-20 nm in thickness. The morphology and structure of such material have been studied by SEM, HRTEM, Raman spectroscopy, XRD, XPS, TGA. In order to investigate the thermal stability of the pristine material, the as-prepared ribbons were annealed at various temperatures in the range 1000 $^{\circ}$C to 2800 $^{\circ}$C. The annealing treatments induced interesting structure changes in the samples, such as defect annihilation and loop formation at the edges, confirmed by HRTEM. A Raman spectroscopy study with many laser energies, enabled us to observe the overall behavior of the main Raman features (D, G, D', G', D+G bands). These phenomena will be discussed in detail. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D25.00002: Nitrogen-induced structures in epitaxial graphene on 6H-SiC(0001) Guofeng Sun, Sung-hyon Rhim, Yun Qi, Michael Weinert, Lian Li Nitrogen-induced structures on epitaxial graphene grown on 6H- SiC(0001) are studied by scanning tunneling microscopy (STM) and first-principles calculations. Several defect structures produced by nitrogen incorporation are observed by STM. Calculations of the energetics of nitrogen substitution at various sites neighboring a carbon vacancy indicate that nitrogen prefers to be at the site nearest to the vacancy, consistent with the STM observations. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:30PM |
D25.00003: Large area, Few Layer Graphene Films on Insulating Substrates Invited Speaker: Graphene has exceptional electronic, thermal and mechanical properties. For the realization of graphene-related applications, it is necessary to develop reliable and low cost fabrication methods of graphene-based structures, ideally on any substrates. In this talk I will present our method of fabricating large area ($\sim $cm$^{2})$ films of single- to few-layer graphene and transferring the films to arbitrary substrates. The graphene films are synthesized by ambient pressure Chemical Vapor Deposition, consist of regions of 1 to $\sim $10 graphene layers and have an average thickness of 2-3 nm. Despite their ultra-thin nature, the films thus produced are continuous over the entire area. Regions of single- or bi-layer graphene with lateral sizes of up to 25 $\mu $m were observed. High Resolution Transmission Electron Microscopy (HRTEM) and electron diffraction revealed that they are crystalline over the entire area and their Raman features were compared to those of graphene derived from mechanical exfoliation of Highly Oriented Pyrolytic Graphite (HOPG). Transistor devices made from these graphene show similar characteristics to ones made from graphitized SiC. Scanning tunneling microscopy imaging reveals interesting Mori\'{e} patterns and helpful insights for the growth of the graphene films on the Ni substrate. The method presented in this work can potentially be scaled to industrial production of graphene films, for applications such as ultra-thin conductive and transparent electrodes, or devices and interconnect for integrated circuits. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D25.00004: Kelvin Probe Microscopy of Single- and Multi-layer Graphene on SiO$_{2}$. Alexandra Curtin, Theresa Swanson, Michael S. Fuhrer Kelvin probe microscopy (KPM) was carried out on mechanically exfoliated graphene samples on SiO$_{2}$ in conjunction with standard atomic force microscopy. Potential differences between the SiO$_{2}$ substrate and graphene flakes were large relative to the average fluctuations over the surface of the graphene. KPM shows a consistent surface potential variation between monolayer, bilayer, and multi-layer graphene, and over folded pleats occasionally found in graphene. The source of these surface potential differences will be discussed. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D25.00005: Evidence of the role of contacts on the observed electron-hole asymmetry in graphene Nimrod Stander, Benjamin Huard, Joseph Sulpizio, David Goldhaber-Gordon Most experiments on graphene devices show a clear difference between the conductances at exactly opposite densities, a phenomenon that has been attributed to different scattering cross sections off charged impurities for opposite carrier polarities. In this talk, we show that properties of the interface between graphene and metal contacts can also lead to such an asymmetry even when bulk graphene transport is particle-hole symmetric. By performing electrical transport measurements in graphene with several sample geometries, with both ``invasive'' probes and ``external'' probes, and with different metal contacts, we associate the asymmetry to \emph{p-n} or \emph{p-p} junctions forming at the interface graphene-metal. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D25.00006: Observation of graphene bubbles and effective mass transport under graphene films Daniil Stolyarov, Elena Stolyarova, Kirill Bolotin, Li Liu, Sunmin Ryu, Kwang Rim, Mark Hybertsen, Igor Pavlishin, Martin Klima, Igor Pogorelsky, Karl Kusche, James Hone, Philip Kim, Horst Stormer, Vitaly Yakimenko, George Flynn Mechanically exfoliated graphene mounted on a SiO$_{2}$/Si substrate was subjected to HF/H$_{2}$O etching or irradiation by energetic protons. In both cases gas was released from the SiO$_{2}$ and accumulated at the graphene/SiO$_{2}$ interface resulting in the formation of ``bubbles'' in the graphene sheet. Formation of these ``bubbles'' demonstrates the robust nature of single layer graphene membranes, which are capable of containing mesoscopic volumes of gas. In addition, effective mass transport at the graphene/SiO$_{2}$ interface has been observed. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D25.00007: Planar tunneling spectroscopy of graphite Richard Jones, Wan Kyu Park, Sam Johnson, Xin Lu, Nadya Mason, Laura Greene The electronic properties of graphite/graphene have become an intriguing area of research in recent years. Probing their electronic density of states (DOS) is of fundamental importance. For this purpose, we choose to do tunneling spectroscopy based on planar junctions. We prepare planar tunnel junctions on graphite using superconducting and normal metal counter-electrodes. An AlO$_{x}$ tunnel barrier is deposited onto a cleaved surface of HOPG using atomic layer deposition, reactive sputtering, thermal oxidation, or plasma oxidation. Differential conductance spectra are taken as a function of temperature down to 4.2K. In general, conductance increases with bias-voltage, which is qualitatively consistent with the predicted DOS in graphite. However, variances in the detailed structures are observed, including a zero-bias conductance dip and multiple peak and hump structures. We will also discuss different growth techniques we propose to yield reproducible junction characteristics. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D25.00008: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:30PM - 4:42PM |
D25.00009: Elasticity and Electron Fractionalization in Graphene William Shively, Dmitri Khveshchenko Much of the recent excitement over graphene comes from the fact that, at half-filling, the energy spectrum may be effectively described by a pair of Dirac fermions, giving rise to a host of effective (2+1)-d chiral gauge theoretic phenomena. In the presence of lattice distortions, hopping electrons bind to topological defects in the honeycomb lattice structure, which can lead to electron fractionalization. Recent work [c.f. Hou et al, PRL \textbf{98} (2007); Jackiw \& Pi, PRL \textbf{98} (2007)] has shown that for Peierls distortions - which in the case of graphene would be described by a Kekul\'{e} lattice dimerization pattern - such fractionalization may occur when electrons interact with topologically-induced vortices. Approaching the problem differently, here we develop a general theory of elasticity for honeycomb lattice structures with various non-trivial dimerization patterns, and explore concomitant possibilities of electron fractionalization. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D25.00010: Electrical Transport in Graphene Hybrid Structures Feng Miao, Wenzhong Bao, Hang Zhang, Chun Ning Lau Graphene, monolayer carbon atoms with honey-comb lattice, has intrigued condensed matter physics field for its unique electrical properties since its first discovery in 2004. The graphene hybrid structures that consist of both single and bi-layers were also experimentally studied recently for its novel properties. We will present our experimental study on the electron transport in graphene hybrid structures and the latest experimental data will be discussed in terms of various theoretic models. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D25.00011: Dependence of the Low Energy Electronic Structure of Multi-layer Graphene on Stacking Order Probed by Infrared Absorption Matthew Sfeir, Kin Fai Mak, James Misewich, Tony Heinz Optical conductivity spectra of multi-layer graphene samples were determined for photon energies in the range of 0.2 -- 1.0 eV. The measurements were performed using synchrotron radiatiaon on well-characterized exfoliated graphene samples on a transparent substrate. We observed distinct optical conductivity spectra for different samples having precisely the same number of layers. In particular, two well-defined types of spectra were obtained in measurements of more than a dozen of four-layer samples. This result can be understood by considering the existence of two stable configurations of four-layer graphene, namely, the ABAB Bernel stacking and the ABCA rhombohedral stacking. The observed absorption features were reproduced by explicit calculations, within a tight-binding model, of the optical conductivities for the two stacking sequences. We have thus shown the possibility of identifying these different crystallographic structures optically. Further, the significant difference found in the low-energy electronic structure suggests that the charge transport behavior of multilayer graphene may also depend on stacking order. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D25.00012: AFM study of the ridge-like network on epitaxial few-layer graphene grown on 4H-SiC [0001] Gyan Prakash, Michael Capano, Michael Bolen, Dmitry Zemlyanov, Ronald Reifenberger Few-layer graphene (FLG) is produced when SiC is heated to temperatures T$>$1475$^{\circ}$C under vacuum conditions. The FLG found on SiC exhibits a 2D mesh of interconnected ridges that extends over many square microns. Smooth regions of FLG are surrounded by the 2D ridges, forming a tile-like surface morphology. The origin of the network is attributed to the compressive stress generated by cooling. For FLG growth at moderately higher temperatures, the thickness of the FLG increases. Under these conditions, AFM studies reveal the emergence of a few well-defined folds that relax the surface stress. In contrast to the previous ridge-like network, the folds appear at only a few locations and have a greater height than the ridges. This AFM study provides insights on how to improve the quality of FLG material grown at elevated temperatures on SiC substrates. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D25.00013: Moir\'{e} Patterns: Fingerprints of Few-Layer Epitaxial Graphene Growth on 4H-SiC(000$\bar{1}$) Laura Biedermann, Michael Bolen, Michael Capano, Dmitry Zemlyanov, Ronald Reifenberger Few-layer epitaxial graphene (FLG) was grown by heating [000$\bar{1}$] silicon carbide to high temperatures (1350--1600$^{\circ}$C) in vacuum. A continuous graphene surface layer was formed at temperatures of 1475$^{\circ}$C and greater. X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used to characterize the quality, thickness, and topography of the FLG. STM studies revealed a wide variety of different nanometer-scale features that include rough graphene, 1D superlattices, grain boundaries, and Moir\'{e} 2D superlattices. Detailed studies of the Moir\'{e} superlattices showed enhanced conductivity due to density of states effects. These Moir\'{e} superlattices also provided insights into the growth mechanisms of FLG on the carbon-face of SiC. \\[0pt] L. Biedermann \textit{et al.}, ``Insights into Few-Layer Epitaxial Graphene Growth on 4H-SiC(000$\bar{1}$) Substrates from STM Studies,'' \textbf{Phys. Rev. B} (submitted). [Preview Abstract] |
Session D26: Focus Session: Computational Nanoscience III: Defects, Doping, and Structure
Sponsoring Units: DMP DCOMPChair: Paul Kent, Oak Ridge National Laboratory
Room: 328
Monday, March 16, 2009 2:30PM - 3:06PM |
D26.00001: Intrinsic magnetism in nonmagnetic nanostructures: Role of localized states and quantum confinement. Invited Speaker: Manipulation of carrier spins in semiconductors for spintronics applications has received much attention recently driven by the promise of new or improved functionalities. This has stimulated extensive research in the area of magnetic semiconductors. Magnetism is traditionally recognized as arising from unpaired electrons in 3d and 4f materials. However, there has been increasing evidence that localized defect states (and/or surface/edge states) in sp materials, especially in some nanostructures, may form local moments and exhibit collective magnetism. In a recent paper [PRL100, 117204 (2008)], we proposed that the duality (i.e., localized vs extended nature) of defect states in wide-gap nitrides and oxides may promote collective magnetism in these materials without magnetic ions. We have recently extended this study to include unexpected magnetism observed in GaN and ZnO nanowires and other artificial quantum structures. Particular attention will be paid to the role of localized states and quantum confinement in promoting unconventional magnetism in these systems. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D26.00002: Defect induced magnetism in semiconductor nanostructures Hyunwook Kwak, Tzu-Liang Chan, James Chelikowsky It has been proposed that magnetic semiconductors can be designed by using non-magnetic defects, e.g., through the introduction of an extrinsic impurity atom that does not exhibit magnetism by itself (Phys. Rev. Lett. 99, 127201). In order to address such proposals, we have employed a real-space pseudopotential method based on the generalized gradient approximation to determine the magnetic properties of boron and aluminum doped silicon nanocrystals and nanowires. We will discuss theoretical evidence for defect induced magnetism as a function of the nanostructure size. We suggest that defect induced magnetism can be strongly enhanced by quantum confinement. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D26.00003: Ferromagnetism driven by extended defects in nanostructured ZnO Aline L. Schoenhalz, Jeverson T. Arantes, Adalberto Fazzio, Gustavo M. Dalpian Spintronic has a particular interest in diluted magnetic semiconductors because these materials present both semiconducting and magnetic properties at the same time. ZnO-based materials and nanostructures have potential applications in this area because they can present room-temperature ferromagnetism when doped with transition metals and, in some cases, spin polarization can be observed even without magnetic impurities. Following recent experimental results reporting this [Nano Lett. 6, 1489 (2007)], we have analyzed several ZnO nanocrystals, with diameters varying from 0.9 to 1.78 nm. Using DFT, we observed that a large amount of surface reconstructions appear in the non-passivated nanocrystals. Depending on the reconstruction, spin polarization without magnetic impurities can be observed at the surface region, what can lead to long-range spin interactions. Thus, we propose that the referred experimental results for nanostructured ZnO can be originated by extended defects such as surfaces. This can also explain the same magnetic behavior presented by non-doped thin films and other ZnO nanostructures. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D26.00004: Self-doping in Boron Nanostructures Hui Tang, Sohrab Ismail-Beigi Boron nanotubes have attracted much attention since their first fabrication in experiments. Boron nanotubes with large radii (R $\ge$ 10 {\AA}) are predicted to be metallic with large densities of states at their Fermi energies, which may provide excellent conducting systems for one-dimensional electronics. In previous work [1], we have shown a class of stable boron sheets, composed of mixtures of triangular and hexagonal motifs, that are likely to be the precursors of boron nanotubes. These sheets are stabilized by a balance of 2-center and 3-center bonding. Here, using density functional theory and Maximally Localized Wannier Functions, we show that adding a boron atom to a boron sheet is equivalent to doping the boron sheet with all three valence electrons of the added atom. Based on this self-doping picture, we propose a simple counting scheme to construct stable boron nanostructures, e.g. from corresponding carbon ones. We also apply this knowledge to study Mg-doped boron sheets and discuss the possible stable structures of MgB$_2$ nanotubes. [1] H. Tang, and S. Ismail-Beigi, Phys. Rev. Lett. 99, 115501 (2007). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D26.00005: Computational Studies of Nanostructures of Boron P. Tandy, M. Yu, C. Leahy, W.Q. Tian, S.Y. Wu, C.S. Jayanthi The goal of this work is to develop a reliable semi-empirical Hamiltonian for boron that may be used to predict nanostructures of boron. It is well known that bonding in boron is complicated as it may form three-center, two-electron bonds. The semi-empirical Hamiltonian used here was recently developed by Leahy \textit{et al}. in the framework of linear combination of atomic orbitals[1]. The salient feature of this Hamiltonian is that it treats environment dependency and charge redistributions on equal footing. It will be shown that such a parameterized Hamiltonian can predict the B$_{80}$ cage structure with C$_{1}$ symmetry as found in a recent first-principles study [2]. Having validated our semi-empirical Hamiltonian for boron with small boron clusters and the B$_{80}$ cage, we have performed a systematic study of other boron nanostructures: (i) larger cage structures ($e.g.$, B$_{215})$, (ii) boron clusters cut from the bulk alpha boron, and (iii) boron sheets (triangular sheets with and without holes). We will discuss the ground state structures of these boron nanostructures as well as the energetics and HOMO-LUMO gaps of different families of boron clusters as a function their diameters. 1. C. Leahy \textit{et al.} Phys. Rev. B74, 155408 (2006). 2. N. G. Szwacki et al. PRL 100, 159901 (2008). [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D26.00006: An efficient method to use \textit{ab-initio} calculations to study substitutional order in nanoparticles Gerbrand Ceder, Tim Mueller To study systems with substitutional disorder researchers commonly use effective Hamiltonians known as cluster expansions.~ In a cluster expansion the phase space of a system is coarse-grained over a fixed set of crystal sites and the energy is expressed as a linear combination of interactions between these sites.~ The coefficients of the linear expansion are typically fit to training data generated using \textit{ab-initio} methods.~ Low-symmetry systems such as nanoparticles require the determination of a large number of distinct coefficients.~ A large amount of training data must be generated for such problems, and the cost of calculating the energy of each training structure is high due to the low symmetry of the system.~ For these reasons it has been impractical to use the cluster expansion to study low-symmetry materials with the same level of accuracy as bulk materials.~ We address this problem by demonstrating new methods that significantly reduce the prediction error of a cluster expansion for a given training set size.~ Our approach makes it possible to study atomic ordering in nanoparticles at a fraction of the current computational cost. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D26.00007: Graph-based global optimization of fully-coordinated cluster geometries Edwin Flikkema, Stefan Bromley We present a detailed global optimization study of cluster geometries with silica nano-clusters (SiO$_2$)$_N$ as a specific example. In an earlier study (Phys. Rev. Lett., 95: 185505, 2005) we used the Basin Hopping methodology combined with an empirical potential to find low-energy cluster geometries. These often exhibit defects such as dangling oxygens. In this contribution we will present an algorithm for global optimization of cluster geometries, which limits the search specifically to fully-coordinated cluster geometries, i.e. defectless clusters where each silicon atom is bonded to 4 oxygen atoms and each oxygen atom is bonded to 2 silicon atoms. This algorithm is based on performing Monte Carlo moves on the set of graphs rather than in coordinate space, the graph being the network of silicon-oxygen bonds. Promising low-energy geometries are selected for refinement using Density Functional Theory calculations. Clusters of a size of up to 30 SiO$_2$ units have been studied. The properties of low-energy fully-coordinated clusters will be compared to those of clusters with defects. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D26.00008: Nanostructure determination from the atomic pair distribution function Luke Granlund, Pavol Juhas, Saurabh Gujarathi, Phil Duxbury, Simon Billinge Many materials at the nanoscale cannot benefit from crystallographic analysis and are unsuitable for refinement techniques that require an initial guess at the structure. One approach to overcoming these difficulties is the Liga algorithm, which generates structures relying solely on distances extracted from the atomic pair distribution function[1,2]. This method is shown to successfully reconstruct the buckyball from experimental data. Recent extensions to multi-component and periodic systems have also allowed reconstruction of common crystals from experimental data. The ability to handle both periodic and nonperiodic cases may make the algorithm a useful tool in the study of local structure deviations in nanocrystals in addition to noncrystalline nanomaterials. [1] P. Juhas, D. M. Cherba, P. M. Duxbury, W. F. Punch, S. J. L. Billinge. Ab initio determination of solid-state nanostructure. Nature, 440, 655-658 (2006). [2] P. Juhas, L. Granlund, P. M. Duxbury, W. F. Punch, S. J. L. Billinge. The Liga algorithm for ab initio determination of nanostructure. Acta Cryst., A64, 631-640 (2008). [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D26.00009: The role of confinement on the diffusion barriers in semiconductor nanocrystals Tzu-Liang Chan, Alexey Zayak, Gustavo Dalpian, James Chelikowsky We find that quantum size effects not only play an important role in the electronic properties of defects in semiconductor nanocrystals, but also strongly affect the incorporation of defect atoms into the nanocrystals. In particular, using ab initio methods based on density functional theory, we predict that Mn defects will be energetically expelled to the surface of CdSe and ZnSe nanocrystals, and that the diffusion barrier of a Mn interstitial defect in a CdSe nanocrystal will be significantly lower than that in the bulk. This can be ascribed to the large surface to volume ratio of nanocrystals, which can effectively release the strain during diffusion. By calculating the vibrational spectrum of the CdSe nanocrystal, we estimated the diffusion rate within the nanocrystal. Our results suggest that energetics can play a role in the self purification of small CdSe and ZnSe nanocrystals, as diffusion of the defect atom can readily occur inside such small nanocrystals. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D26.00010: Accelerated Kinetic Monte Carlo Simulations of Vacancy-Mediated Arsenic Diffusion and Clustering in Silicon Brian Puchala, Michael Falk, Krishna Garikipati During semiconductor device fabrication, ion implantation of dopants creates large populations of defects, vacancies and interstitials, which mediate dopant diffusion. Experiments have shown large changes in dopant diffusivity in silicon as a function of annealing time and dopant concentration. We perform kinetic Monte Carlo (KMC) simulations of vacancy-mediated arsenic diffusion in silicon to investigate the effect of dopant concentration, distribution and clustering on diffusivity. In order to follow the diffusion and breakup of clusters, on the order of minutes, our KMC simulations are accelerated using absorbing Markov chain analysis on states intelligently chosen on-the-fly to fill trapping basins in the local energy landscape. At lower dopant concentrations, we calculate the diffusivity and breakup rates of different cluster types and a mean field approach can be used to describe the overall cluster population evolution and dopant diffusivity. Above a critical concentration this mean field description fails as dopants become close enough to form a percolating structure throughout the material. At all concentrations, diffusivity decreases significantly over time as larger, less mobile clusters form. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D26.00011: First Principles Study of the Effect of Lattice Strain on Diffusion Barriers Handan Yildirim, Talat S. Rahman To understand the three times larger values of the Erlich Schoewebel barriers for Ag on Cu(100) as compared to that for Cu on Ag(100) as obtained from our density functional theory calculations, we performed the diffusion of Ag adatom on Ag(100) and Cu adatom on Cu(100) under uniform lattice strain of 0-5{\%}. We find the origin of the differences in the energetics to be the combination of Cu-Ag electronic coupling, relative atomic sizes and adsorbate-substrate lattice mismatch. The diffusion barriers on the ideal surfaces are found to decrease with increasing compressive strain and to increase with increasing tensile strain and almost a linear function of strain up to 5{\%}. We show that this trend is universal and transferable to other metals. We will discuss the consequences of the modifications in the height of the ES barriers on growth modes. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D26.00012: Quantum confinement in P-doped Si[110] nanowires Jiaxin Han, Tzu-Liang Chan, James Chelikowsky Recently, doped Si nanowires have been synthesized and demonstrated experimentally that they can be used as interconnects in electronic circuits or building blocks for semiconductor nanodevices. In order to understand how doping operates at the nanoscale, we used a real-space first-principles pseudopotential method to study P-doped Si[110] nanowires. We examined the size dependence of the electronic binding energy for the P donor level. We found the donor electron to be more strongly bound to the P atom with decreasing nanowire diameter owing to quantum confinement. We also examined the energetically favorable position of the P atom in Si nanowires. For nanowires with diameter less than 1 nm, the P atom is expelled to the surface owing to the stress introduced by the defect, which suggests that doping will be difficult for small-diameter Si nanowires. In addition, we calculated the core-level binding energy shift as the P atom moves from the surface towards the center of the nanowire. We will compare our results with experimental measurements. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D26.00013: Effect of structure, surface passivation, and doping on the electronic and optical properties of GaAs nanowires: A first principles study. S. V. Khare, V. Gade, N. Shi, R. Ramprasad We investigate the structural, energetic, electronic, and optical properties of hydrogen-passivated doped and undoped gallium arsenide nanowires along [001], [110], and [111] directions with diameter d up to 3 nm, using \textit{ab initio} methods. A critical diameter d$_{c}\approx $2 nm is found above which all wires have faceted cross sections determined by the symmetry of their axis. The wires possess several electronic properties relevant for sensing and other nanoelectronic applications: (i) Quantum confinement has a substantial effect on the electronic band structure and hence the band gap (E$_{g})$, which increases with decreasing diameter. (ii) Unlike Si or Ge wires, GaAs wires oriented along all axes are found to have a direct E$_{g}$.$_{ }$(iii) The electronic band structure shows a significant response to changes in surface passivation with hydrogen. (iv) Doping of wires with n and p type atoms produced a response in the band structure similar to that in a doped bulk crystal. (v) However, the dielectric function shows differences in absorption peaks with p type versus n type doping. [Preview Abstract] |
Session D27: Advances in Instrumentation and Measurement I
Sponsoring Units: GIMSChair: Larry Rubin, MIT
Room: 329
Monday, March 16, 2009 2:30PM - 2:42PM |
D27.00001: Mechanical properties of silicon nanowires studied by polarization-enhanced fiber-optic interferometry John Nichol, Eric Hemesath, Lincoln Lauhon, Raffi Budakian Silicon nanowires have recently attracted attention as promising force sensors due to their inherent low dissipation and high frequency. One of the principal challenges to the use of nanowires as scanning probe force sensors is displacement detection. By exploiting the polarization anisotropy in light scattering from single nanowires, we have used fiber-optic interferometry to detect the displacement of individual silicon nanowires. We achieve a displacement sensitivity of 0.5~pm/$\surd $Hz for 15~$\mu $W of light incident on the nanowire. The nanowires studied have ultralow mechanical dissipation in the range of 2$\times $10$^{-15}$-~2$\times $10$^{-14}$~kg/s. We also discuss the effects of hydrogen surface passivation on mechanical dissipation. Further progress toward the use of nanowires as scanning probe force sensors is discussed. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D27.00002: Zero Flux Anomaly in Mesoscopic Normal Metals Julie Bert, Hendrik Bluhm, Nicholas Koshnick, Martin Huber, Kathryn Moler Our recent scanning SQUID susceptometry measurements of individual mesoscopic normal metal rings found persistent currents consistent with theoretical predictions. In addition to the persistent current signal, all rings exhibited a step anomaly in the SQUID response vs flux around zero applied flux and a large paramagnetic linear susceptibility. We present the characteristics of the zero flux anomaly observed in multiple rings and thin films and explore possible explanations. Key features include a notable frequency dependence between 11 and 333 Hz and a strong spatial correlation with the large paramagnetic spin signal. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D27.00003: Second Harmonic Technique for Thermal Conductivity Measurement in a Pulsed Magnetic Field Yoko Suzuki, Jonathan B. Betts, Albert Migliori We describe a second-harmonic technique to be used eventually to probe the thermal conductivity of LSCO with superconductivity suppressed by high magnetic fields. The technique is suitable for the high-noise environment of pulsed magnets. Unlike the 3$\omega $ technique, a heater and a thermometer are mounted separately. Therefore, the 2$\omega $ signal is the dominant signal in the thermometer output. The frequencies are chosen so that the thermal penetration depth is smaller than the sample thickness. The thermometer response time and thermal impedance associated with material interfaces are carefully tested and compared to calculation. The calculations are based on exact solutions of the full bulk heat transport equations and produce results different from the lumped-constant approximations often used in ac calorimetry. Work performed under the auspices of the National High Magnetic Field Laboratory. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D27.00004: Nanoscale imaging magnetometry with single spins in diamond Gopalakrishnan Balasubramanian, Julia Tisler, Roman Kolesov, Fedor Jelezko, Joerg Wrachtrup Single Nitrogen-Vacancy colour centers in diamond are gaining popularity because of its exceptional optical and spin properties. The single spin of the defect can be manipulated optically, providing a efficient way to entangle single electron spins and couple nuclear spins qubits in diamond.[1] Long spin coherence time of these single defects finds application as sensitive magnetic field probes. Using engineered diamond we can achieve ultrahigh sensitivity using which we will be able to detect a single external electron or nucelar spin.[2] Controlled creation of these color centers inside nanodiamonds offers diverse applications. By attaching these single spins to the tip of a scanning probe, we were able to perform sensitive scanning probe magnetometry at nanoscale.[3] Improving this device by using quantum grade diamond and synchronized NMR pulse sequences we would have the ability to perform nanoscale NMR/MRI of single molecules.\\[0pt] [1] Neumann, P. et al. Multipartite Entanglement Among Single Spins in Diamond. Science 320, 1326-1329 (2008).\\[0pt] [2] Maze, J. R. et al. Nanoscale magnetic sensing with an individual electronic spin in diamond. Nature 455, 644-647(2008).\\[0pt] [3] Balasubramanian, G. et al. Nanoscale imaging magnetometry with diamond spins under ambient conditions. Nature 455, 648-651(2008). [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D27.00005: Elastic cotunneling through a quantum dot in the presence of electromagnetic fluctuations Vladimir Bubanja We consider the effect of electromagnetic fluctuations on electron transport through a quantum dot in the Coulomb blockade regime. We obtain the analytic expression for the elastic cotunneling current which shows that the electromagnetic fluctuations cause the power law suppression at low voltages, $I \sim V^{1+2 R/R_{K}}$ where $R$ is the Ohmic part of the circuit impedance and $R_{K}$ is the quantum resistance. This elastic cotunneling current is proportional to the level spacing of the dot and is the dominant transport below certain cross-over voltage, above which inelastic cotunneling dominates. Both cotunneling processes are of importance in accuracy considerations of the operation of the R- pump, which is a single-electron tunneling device that is of interest for applications in electrical metrology. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D27.00006: Graphenated Infrared Screens: A New Platform for Bio- Detection. Amrita Banerjee, Dieter Moeller, Haim Grebel We are proposing a novel spectroscopic tool -- the graphenated infrared (IR) screens. It is aimed to enhance weak IR and Raman signals. Metallo-dielectric screens have been used for astronomy and remote sensing applications. These periodic structures are at resonance with the IR wavelength of interest: a standing wave of surface charges is formed at resonance conditions, which enables transmission or, reflection of certain IR bands. Graphene is a monolayer thick crystal of carbon. It is chemically inert and exhibits very large mobility values. Recently, we succeeded in fabricating mono and a few-layered suspended graphene on top of these IR screens. The result is a new spectroscopic platform, which enhances weak IR and Raman signals of molecules and specifically, bio-species, which are residing on the graphene layer. The IR absorption and Raman signals of bio-species under test have exhibited strong dependence on the screen periodicity pitch as well as on its orientation. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D27.00007: Development of a Microfocus Beamline for Angle-Resolved Photoelectron Spectroscopy T. Miller, M. Bissen, T.-C. Chiang Synchrotron-based angle-resolved photoemission spectroscopy has proven to be a powerful tool in the elucidation of electronic structure of solids. The technique is now being applied to a wide variety of materials, and the macroscopic sampling area has become a limitation. For example, cleaving may expose different crystal planes, and the area covered by the incident photon beam may then consist of a collection of small domains with different photoemission spectra. The result is an average which obscures the true nature of the material. For this reason a beamline with a small focus has been proposed to be used with an angle-resolved photoemission endstation at the Synchrotron Radiation Center in Stoughton, WI. Reflective optics would be used to produce a microfocus at the sample of the light from an undulator beamline, providing submicron spatial resolution, while electron emission angles and energies are measured using an imaging electron energy analyser. The microfocusing optics and possible applications will be discussed. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D27.00008: Near field emission scanning electron microscopy Taryl Kirk, Lorenzo De Pietro, Olivier Scholder, Thomas Baehler, Urs Ramsperger, Danilo Pescia We present a simple ``near field emission scanning electron microscope'' (NFESEM) capable of imaging conducting surfaces with high spatial resolution. In this instrument electrons are excited from the sample surface after undergoing interactions with a low-voltage ($<$ 60V) primary beam of electrons field-emitted from a Tungsten tip positioned tens of nanometers above the sample. Topographic images, determined from the intensity variations of secondary and backscattered electrons, yield a vertical resolution on an atomic scale and a lateral resolution of less than two nanometers. We report on the first topographic electron intensity images of terraces and mono-atomic steps on a single crystal substrate, not yet attained with a remote electron gun in conventional scanning electron microscopy. The topographic contrast of the extracted electrons and the field emission (FE) current are indistinguishable, in agreement with theoretical models of optimal spatial resolution. We assert that additional analysis of the secondary electrons will also exhibit a comparable resolution. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D27.00009: Particle Characterization using Holographic video Microscopy Foo Chiong Cheong, David Grier In-line holographic video microscopy can be interpreted with Lorenz-Mie theory to obtain exceptionally precise measurements of individual colloidal spheres' dimensions and optical properties, while simultaneously tracking their three dimensional motions with nanometer-scale spatial resolution at video rates. This method works over the entire range of particle sizes and compositions for which Mie scattering theory applies. Unlike other light scattering techniques for measuring particle size or refractive index, holographic particle analysis can be applied directly to individual particles. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D27.00010: Three-dimensional position determination of nanoparticles using a two-photon microscope James Germann, Lloyd Davis, Brian Canfield, Alexander Terekhov We are developing a means to extend the two-photon microscope to enable three-dimensional sub-diffraction measurement of the position and trajectory of a single nanoparticle as it traverses the probe volume. By use of a Ti-Sapphire laser and a double-Mach-Zehnder interferometer configuration, four laser beams with temporally interleaved pulses are created. These are tightly focused by a 1.2-NA water-immersion microscope objective to four overlapping volumes centered at slightly offset points arranged in a tetrahedron. Fluorescence from the four-focus probe volume is then collected onto a single-photon avalanche diode and the photon time stamps are recorded. Time-resolved photon detection with maximum-likelihood analysis is thereby used to determine the position of the nanoparticle from the relative intensities of fluorescence from each of the four foci.~ We present measurements of the profile of the four-focus configuration and results from calibration experiments obtained by translating a single gold nanodot or a fluorescent nanobead through the probe volume. Application of the position determination to single-particle trapping is also briefly discussed. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D27.00011: Capabilities of high-sensitivity spectral fluorescence-lifetime imaging for resolving spectroscopically overlapping species Justin Crawford, Lloyd Davis, Brian Canfield The ability to separate the contributions from spectroscopically overlapping fluorophores has enabled significant breakthroughs in cellular imaging. However, commercial microscopes for this purpose generally use analog light detection with least-squares curve-fitting analysis. Improvements in sensitivity are possible and will lead to new applications. To this end, we have constructed a microscope with a high-throughput Brewster-prism spectrometer and four high-quantum efficiency single-photon detectors, coupled with time-correlated single photon counting electronics to provide added temporal resolution. We have demonstrated the use of maximum-likelihood (ML) methods for analyzing small numbers of photons to find the contributions from fluorescent species with differences in excitation and emission spectra. However, it is difficult to resolve fluorophores with different temporal decay profiles because the single-photon counting modules exhibit a count-rate-dependent time-walk. We discuss extension of the ML-analysis to account for a varying time-walk and results from Monte Carlo simulations to ascertain the minimum number of photons needed to reliably resolve specific fluorophores. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D27.00012: Development of a low-cost small-sized scanning transmission ion microscope of moderate resolution with educational and other potential applications Arthur Pallone Scanning transmission ion microscopy (STIM) has applications in many fields of study such as materials and device engineering, biological and geological sciences, and the arts. Since STIM is practiced at ion beam facilities, many persons outside of the ion beam community who could benefit from STIM are unaware of its potential. In an effort to better educate the public about STIM, an inexpensive portable demonstration unit suitable for interactive classroom use and public outreach events is under development. The required parts are readily available, mostly at local electronics and office supply stores. Progress toward completion of the demonstration unit and future efforts to modify the unit to support thin film research will be discussed. Activities that demonstrate the three modes of STIM will also be presented. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D27.00013: A two dimensional piezoeletric micro-positioner K.-W. Ng, John Nichols, J. W. Brill A scanning probe microscope can provide very high resolution imaging, but only within a small scanning area. There is a high demand for compact long range positioners, so that distant locations on the same sample can be imaged and studied. We will present information on the design and operation of a piezoelectric driven two-dimensional micropositioner that can provide long range motion in the x- and z-directions. The z-direction motion can be used for coarse approach, while the x-direction motion can be used to scan along the sample surface. The device is build as one single unit, so it is extremely compact and rigid, and can provide a high resonance frequency platform for high performance scanning probe microscopy. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D27.00014: High Speed Scanning Property Mapping ($>$1 frame per second) Bryan Huey, Nicholas Polomoff, Atif Rakin, Vincent Palumbo Atomic Force Microscopy is coupled with concepts of acoustics to achieve nanoscale property contrast at line scanning rates approaching several kHz. This allows novel measurements of surface dynamics, efficient large area imaging, and high throughput experiments with SPM. Examples for mechanical contrast on block copolymers, semiconductors, and eutectic alloys are included, as well as high speed electric and magnetic field imaging. Coupled electromechanical contrast (piezoelectric) is also employed with PZT thin films to uniquely monitor ferroelectric domain dynamics. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D27.00015: Method for Detecting Position and Orientation of Convex Objects in 3D Scans Alexander Jaoshvili, Paul Chaikin We have developed an algorithm for detecting the center positions and orientations of mono-disperse objects which pack a container from the data collected in a 3 dimensional scan such as obtained by MRI. The algorithm is applied to a variety of geometrical convex shapes including, ellipsoids, cubes and tetrahedrons. From the positions and orientations we are able to reconstruct the number and type of interparticle contacts and constraints and thus to test Maxwell's Isostatic conjecture in the case of ``random close packing.'' We also obtain translational and orientational correlation functions. [Preview Abstract] |
Session D28: Focus Session: Thermoelectric Materials: Oxides and Complex Crystals
Sponsoring Units: DMP FIAPChair: Ali Shakouri, University of California, Santa Cruz
Room: 330
Monday, March 16, 2009 2:30PM - 3:06PM |
D28.00001: Thermoelectricity in oxides and weakly coupled single molecules Invited Speaker: Complex oxides have emerged as promising candidate materials for thermoelectric and energy applications. The study of charge and heat transport in these systems is also very interesting and important from the point of view of fundamental physics. We show that oxides in the narrow-bandwidth limit have high values of the thermopower and power factor and also violate the Wiedemann-Franz law yielding high values of the electronic part of the figure of merit. These theoretical results agree with the data on Na$_x$CoO$_2$. We argue that in another oxide Sr$_ {1-x}$La$_x$TiO$_{3-\delta}$, a large effective mass (small bandwidth) band appears due to oxygen vacancies, which can be exploited for thermoelectric applications and comment on recent experiments. Finally, we show that there are commonalities in the thermoelectric behavior of narrow bandwidth oxides and weakly coupled single molecules. The latter systems also offer promise as thermolectric materials due to the possibility of large values of thermopower and we comment on the limiting effect of phonons on their figures of merit. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D28.00002: Magneto-Spectroscopic Measurements of Ca$_{3}$Co$_{4}$O$_{9}$ Thin Films and Single Crystals Jiufeng Tu, Dimitar Dimitrov, Weidong Si, Qiang Li In recent years, the 2D-layered cobaltates have emerged as promising p-type thermoelectric materials due to their unique combinations of high thermo-coefficient and good metallic transport properties at ambient temperatures. These systems show high thermoelectric figure of merit and are ideal candidates as the materials of choice at elevated temperatures. We have carried out far-infrared magneto- spectroscopic studies of Ca$_{3}$Co$_{4}$O$_{9}$ thin films as a function of frequency, magnetic field and temperature with the emphasis on the coupling between the lattice, the charge and the spin degrees of freedom. The spectral response is different with magnetic field perpendicular or parallel to the CoO$_{2}$ layers. Below 20K, hysterisis occurs for perpendicular field but not for parallel field. This indicates that the negative magneto-resistance is due to reduced magnetic scattering when Co spins become aligned. Our results are consistent with co-existence of two types of carriers. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D28.00003: Direct measurement of charge transfer and spin stat transitions in thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$ Guang Yang, Quentin Ramasse, Robert Klie The misfit-layered thermoelectric material Ca$_{3}$Co$_{4}$O$_{9}$ has been the focus of many recent studies due to its high thermal power and good high temperature stability. In particular, it has been suggested that the presence of a mixed valence state in the strongly correlated CoO$_{2}$ layer is essential for the high p-type thermoelectric properties in Ca$_{3}$Co$_{4}$O$_{9.}$ In this study, we combine aberration-corrected scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to study the atomic and electronic structures of Ca$_{3}$Co$_{4}$O$_{9}$. We will show that the position of the O atomic columns in the CoO$_{2}$ layers are highly ordered and can therefore be directly imaged, while the CoO columns in the Ca$_{2}$CoO$_{3}$ rocksalt layer exhibit a strong modulation in the (010) direction. Further, we measure the local Co valence and find significant hole transfer from the rocksalt CoO to the strongly correlated CoO$_{2}$ layers. In addition, we will present the results of our in-situ heating experiments of Ca$_{3}$Co$_{4}$O$_{9}$ [010] at 500 K, which show that the phase transition at 420 K is not accompanied by a structural transition but rather a transition of the Co-ion spin states. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D28.00004: Thermoelectric properties of doubly doped Strontium Titanate thin films Jayakanth Ravichandran, Matthew L. Scullin, Subroto Mukerjee, Joel Moore, R. Ramesh, Arun Majumdar Lanthanum doped Strontium Titanate (SrTiO$_{3})$ is amongst the most promising n-type thermoelectric materials for power generation. We report a double doping method for thin films of SrTiO$_{3}$, grown on (001) oriented LSAT substrates by Pulsed Laser Deposition (PLD), where doping of SrTiO$_{3}$ in the A-site by Lanthanum is accompanied by doping with oxygen vacancies. Based on careful transport measurements, we show that it is possible to obtain enhanced thermoelectric power factor in the limit of high effective mass and large carrier concentration in these thin films. The presence of oxygen vacancies also serves to decrease the thermal conductivity due to effective phonon scattering. The optimized doping concentration leads to a thermoelectric figure of merit, zT $>$ 0.2 at room temperature. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D28.00005: Thermoelectric Properties and Band Structure Calculations of Novel Boron Network Compounds Takao Mori, Toshiyuki Nishimura, Yuri Grin, Toetsu Shishido, Kazuo Nakajima Boron is an interesting element, tending to form atomic networks such as 2D atomic nets and clusters, with some analogy to carbon systems which have been more extensively studied. Boron has one less electron than carbon and thus is electron deficient when forming atomic networks, but this causes it to have a special affinity with the rare earth elements and as a result, many new compounds have recently been discovered [1]. Their potential as viable thermoelectric materials is attracting interest since they are high-temperature materials and possess intrinsic low thermal conductivity, with some compounds exhibiting Seebeck coefficients in excess of 200 $\rm \mu$V/K above 1000 K. The thermoelectric properties and band structure calculations of novel borides such as RB$_{44}$Si$_{2}$, RB$_{17}$CN, RB$_{22}$C$_{2}$N, RB$_{28.5}$C$_{4}$ will be presented. Features in the band structure near the Fermi level indicate large doping effects in these compounds. Various doping experiments were carried out resulting in large increases to the figure of merit. [1] T. Mori, ``Higher Borides,'' in: {\it Handbook on the Physics and Chemistry of Rare Earths}, Vol. 38, (North-Holland, Amsterdam, 2008) p. 105-173. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D28.00006: Electronic structure and high-temperature properties of doped Hf$_{0.5}$Zr$_{0.5}$CoSb phases Jack Simonson, Slade Culp, S. Joseph Poon, Vijayabharathi Ponnambalam, Justine Edwards, Terry Tritt Half-Heusler alloys with compositions of the form Hf$_{0.50}$Zr$_{0.50}$CoSb were synthesized with Mn substituted to one or both of the Hf/Zr and Co sites or with the Sb site doped with Sn. The thermoelectric properties were evaluated from 300 K to 1000 K. The introduction of Mn was performed to investigate modifications to the band structure near the Fermi energy caused by transition metal substitution. Mn substitutions were discovered to increase the electrical resistivity dramatically while having no beneficial impact upon the thermopower. The Sb-doped alloys, on the other hand, exhibited lowered resistivity and thus increased efficiency of high temperature power generation. The results of both substitutions will be discussed in light of recent first-principles electronic structure calculations. In the Sb-doped alloys, ZT was found to reach 0.5 at 1000 K and is projected to increase to 0.6 K at 1100 K, surpassing the industry standard for p-type materials as set by SiGe alloys. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D28.00007: Thermoelectric power generation in ternary skutterudites: a first-principles Wannier-functions' study Dmitri Volja, Marco Fornari, Boris Kozinsky, Nicola Marzari We study from first-principles ternary skutterudites derived from $CoSb_3$, where the pnictogen is substituted with elements from the IVB and VIB groups. We focus on $CoGe_{3/2}S_{3/2}$, $CoGe_{3/2}Te_{3/2}$ and $CoSn_ {3/2}Te_{3/2}$, and compute the structure, electronic structure and vibrational properties from density-functional and density-functional perturbation theory. Since the direct evaluation of transport quantities in the relaxation-time approximation is computationally demanding, we use maximally-localized Wannier functions (MLWFs) for accurate integrations of operators across the Brillouin zone. This MLWFs basis leads to a very efficient and well-conditioned scheme to calculate the thermoelectric transport coefficients and to disentangle and identify the contribution of single bands. In addition, it provides a detailed, transferable picture of bonding in these complex materials. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D28.00008: Thermoelectric Properties in Nanostructured p-type Skutterudites Xiao Yan, Qing Hao, Jian Yang, Hui Wang, Yucheng Lan, Dezhi Wang, Gang Chen, Zhifeng Ren Skutterudites are good examples of phonon glass electron crystal (PGEC), which is proposed to be one of the most desirable materials to maximize the thermoelectric figure of merit. The skutterudite structure has two voids in each unit cell that are large enough to accommodate a variety of atoms, such as La, Ce, Nd, Sm, Yb, etc. These atomic void-fillers rattle about in their oversized cages, thereby drastically reducing thermal conductivity and maximizing ZT. My work on p-type skutterudites is based on compounds of a general formula RFe$_{3.5 }$Co$_{0.5}$Sb$_{12}$ where R stands for a void filler. Besides the influence of rattling of the void fillers, thermal conductivity can be further depressed by the increased phonon scattering at the increased grain boundaries due to nano size grains. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D28.00009: Calculation of electric field gradients and site preference in Ba-Al-Ge clathrates Sergio Rodriguez, Weiping Gou, Joseph Ross Sn, Ge or Si can form cage-like clathrate structures, many of which exhibit enhanced thermoelectric performance. To understand Al substitutional occupation in Ba$_8$Ge$_{46-x-y}$Al$_x\Box_y$ clathrates we performed NMR lineshape simulations for $3 \leq x \leq 24$, and $0 \leq y \leq 3$, where $\Box$ represents a vacancy. The electric field gradient (EFG) was calculated for Al sites assuming an ordered superstructure. To obtain the EFG we used $ab$ $initio$ methods in the Generalized Gradient Approximation as implemented by the WIEN2k code with structural relaxation. Results where used to simulate NMR lineshapes numerically. These were compared to our previously reported NMR lineshapes. For fully occupied Ba$_{8}$Ge$_{30}$Al$_{16}$ we compare different site occupations, obtaining good agreement and thus information about Al site preferences. For reduced-Al samples, WDS measurements indicate the presence of spontaneous vacancies. In the case of the Zintl phase Ba$_{8}$Ge$_{33}$Al$_{12}\Box_1$ we found that Al sites adjacent to the vacancy exhibit a large EFG, while those with the vacancy further away have smaller EFG's. Assuming a larger Knight shift for sites next to vacancies, we obtain good agreement with NMR experimental results for reduced-Al Ba$_{8}$Ge$_{34}$Al$_{12}$. We infer that Al prefers locations close to vacancies rather than random occupation. Supported by Robert A. Welch Foundation (Grant A-1526). [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D28.00010: The effects of silicon on the electronic properties of the clathrates A$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ (A= Ba, Sr) Emmanuel Nenghabi, Charles Myles We have studied the structural and electronic properties of the Ba and Sr guest-containing type-I semiconductor clathrates Ba$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ and Sr$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ for Si compositions x = 0, 5 and 15. Our calculations are based on the generalized gradient approximation (GGA). Starting with stable structures of the Ge-based Type I clathrate semiconductors Ba$_{8}$Ga$_{16}$Ge$_{30}$ and Sr$_{8}$Ga$_{16}$Ge$_{30}$ containing no Ga-Ga bonds, we have constructed unit cells of Ba$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ and Sr$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ by replacing appropriate numbers of the framework Ge atoms with Si. For the values of x that we have considered, we find that the fundamental band gap of Ba$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ decreases with increasing x but that the band gap of Sr$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x }$ increases with increasing x. We also find that several electronic states near the top of the valence band and near the bottom of the conduction band in both materials are modified by the Si p states. The trends in the structural and electronic properties of these materials as x is varied are discussed, and our results are compared to experiment where possible. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D28.00011: Effects of silicon on the vibrational and thermal properties of the clathrates A$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ (A= Ba, Sr) Charles Myles, Emmanuel Nenghabi Using the GGA, we have calculated the vibrational and thermal properties of the clathrates Ba$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ and Sr$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ for x = 0, 5 and 15. We find that the Ba and Sr guests have localized vibrational modes lying below 80 cm$^{-1}$, which tend to reduce the host acoustic bandwidth. We predict that there is an upshift in the framework optic modes as x increases and that the guest-atom Einstein temperatures vary with x. Our predicted isotropic atomic displacement parameters as functions of temperature for the Ba and Sr guests in these clathrates show that Sr has a larger isotropic displacement parameter than Ba, suggesting that Sr should be more efficient than Ba in lowering the thermal conductivity. We also predict the temperature dependences of the vibrational specific heat, the entropy, and the Helmholtz free energy in these materials. We find that the specific heat increases smoothly with temperature and approaches the Dulong-Petit value at room temperature. We also find that there is a slight x dependence of the heat capacity, free energy, and vibrational entropy. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D28.00012: Large Thermoelectric power factor in CrN Camilo Quintela, Francisco Rivadulla, Jose Rivas We report the electrical resistivitiy and thermoelectric power of stoichiometric and hole-doped chromium nitride (CrN). The results indicate a considerably large power factor, of the order of 1(microW/cmK2) at 400 K, increasing with temperature. Hall effect measurements were used to elucidate the mechanism of electronic transport in this system, in order to optimize its properties. The easy of grow in the form of nanoparticles and thin films, along with a good thermal stability up to 700 K, could make this material interesting for applications at moderate temperatures. [Preview Abstract] |
Session D29: Focus Session: The Physics and Astronomy New Faculty Workshops I
Sponsoring Units: FEdChair: Robert Hilborn, University of Texas at Dallas
Room: 333
Monday, March 16, 2009 2:30PM - 3:06PM |
D29.00001: A Dozen Years and a Thousand Participants: The Workshops for Preparing New Faculty in Physics and Astronomy Invited Speaker: Beginning in 1996, an annual workshop for newly hired faculty in physics and astronomy has been held under the organizational leadership of AAPT, APS, and AAS. To date more than 1000 faculty have participated in this workshop, representing approximately 25{\%} of the new hires at all U. S. institutions that award a baccalaureate in physics or astronomy, from 4-year colleges through research universities. The original motivation for the workshops was to improve physics teaching by introducing new faculty to instructional strategies and innovations that had been shown to be effective in a variety of contexts. The need for such a program was suggested in part by the belief that a national mentoring workshop could effectively address a commonality of physics and astronomy teaching challenges that transcended institutional characters and types, and also in part by the reaction to a significant decrease in the number of baccalaureate physics degrees awarded in the U. S. in the 1990s, which many believed was due to ineffective and uninspiring teaching at the undergraduate level and especially in introductory courses. Based on surveys of the participants (and their department chairs), we have found that a large fraction of the participants have become adopters of innovative teaching techniques and that they rate the workshops as the most significant cause of the improvements in their teaching. This talk will summarize the development of the workshop program since its inception, the measures of its success in improving teaching, and the plans for its future. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D29.00002: Teaching New Dogs New Tricks: The Next Generation of Physics Faculty Gerald Feldman I was privileged to attend the first New Faculty Workshop (NFW) in 1996, and that was the spark that kindled my keen interest in teaching methodology, learning assessment, and Physics Education Research (PER). Following Eric Mazur's introduction to Peer Instruction, I became a strong advocate of \textit{ConcepTests} in class, implemented an electronic student response system before they were widely in vogue, and emphasized conceptual understanding on the same footing as numerical computations. This led to a research project on the efficacy of in-class \textit{ConcepTests}, and further, to the linking of conceptual and numerical aspects in a ``thinking skills'' curriculum for introductory physics at George Washington University (GWU). After ``dabbling'' in PER for some years, several of us at GWU now have a credible research program and our first Ph.D. student in PER. The methods espoused by PER have extended to other members of the Physics Department, and even beyond, to other science departments on campus. Following this trend, we have most recently (Spring 2008) implemented a SCALEUP collaborative classroom modeled after the work of Bob Beichner at NC State. Overall, the teaching climate in the GWU Physics Department has changed considerably over the past decade, and it is clear that these changes ultimately trace their origins back to the impact of the NFW on the faculty members who have attended over the years. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D29.00003: Effects of the New Faculty Workshop at WFU Freddie Salsbury Two members of the Wake Forest University Physics department have attended the Physics and Astronomy New Faculty Workshop and have implemented many of the lessons learned in their classes and in the department. In particular, the impacts on teaching introductory and upper-level physics, and developing a biophysics track will be discussed. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D29.00004: Twenty semesters later: how NFW helped us to multiply majors A. Alan Middleton In the Fall of 1997, I missed my daughter's first Halloween to attend the NFW. Though that was disappointing, I was pleasantly surprised to learn that there were a number of people who thought about teaching and had very constructive ideas to share. I immediately applied what I had learned in the large lecture class that I taught for a few years, with success for my students and my career. I then spent 7.5 years as the Director of Undergraduate Studies: the contacts and insights provided by the NFW and subsequent occasional attendance at AAPT meetings were most helpful in improving our program (the number of graduating majors increased from about 3 to over 15 per year), along with the help of a few dedicated faculty at Syracuse. The specific pedagogical viewpoints and tools introduced at the NFW were helpful in my teaching and administrative work, but the most useful part of the experience was the license granted to seek improvements in both teaching and the undergraduate program and providing contact with others who took such issues seriously. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D29.00005: The Best of Both Worlds Elizabeth H. Simmons Thanks to the New FacultyWorkshop, I now have dual identities: as a professor in a research-focused university physics department and as Dean of a teaching-focused undergraduate residential science college within that university (Lyman Briggs College). I'll talk first about how the NFW changed my perspective on education and outreach -- and how that affected the climate in the department I was in at the time. Next, I'll comment on how this shift of perspective eventually led to new professional opportunities, including my current dual position. The lessons from the NFW have contributed directly to my acquiring the skills required of a leader in an interdisciplinary college that encourages its faculty both to be active disciplinary researchers and to take a scholarly approach to teaching. Finally, I'll mention how being part of the college's community of teacher-scholars has helped my teaching to continue evolving in directions compatible with the aims of the New Faculty Workshop. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D29.00006: ReDUCE: Rethinking Directions in Undergraduate Curriculum Experiences Carlos Wexler, Deborah Hanuscin, Matthew Mower, Haskell Taub There is a major emphasis in higher education on rethinking undergraduate science instruction, particu-larly in introductory courses. In Fall/2007 a collaborative team at the University of Missouri formed and was given a ``green light'' to start an overhaul of the curriculum and instruction of (algebra-based) Physics I and the associated laboratory/recitation, a course taken by $>$ 500 students/yr. Earlier, we had identified problems with the current status of the course: the number of topics ``crammed into the syllabus,'' disconnectedness of laboratories to each other, and lack of conceptual coherence between laboratory, recitations and lectures. In Spring/2008, the group was awarded an interdisciplinary grant from the College of Education to begin work on realigning the lecture, lab, and recitation components of the course around a coherent curriculum that builds towards ``big ideas'' in the discipline (a ``narrow but deep'' approach). In Fall/2008 we implemented the first pilot test by combining exploratory labs, ``take-home-minilabs,'' and the use of Tutorials (in one section of the lab of this course, ca. 20 students). In this talk I will discuss early results and conclusions of this experiment, the next steps in the academic transformation, funding issues, and hurdles faced towards implementation on larger scale. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D29.00007: JiTT and Peer Instruction in the General Physics Sequence at Dominican College: A One-Year Retrospective Kathleen Hinge Energized by the New Faculty Workshop of November, 2007, the author returned to Dominican College of Blauvelt with dreams of colossal gains on Force Concept Inventory (FCI) scores. Replicating the reported successes at Harvard and the US Air Force Academy would be difficult at Dominican: More than one-third of Dominican's physics students are minority, and an even greater portion of students come from secondary education without the ability to synthetically apply concepts. Undeterred, the author was committed to overhauling the General Physics sequence around NFW tenets. Beginning in January, 2008, material presentation went from traditional lecture format to a combination of Just in Time Teaching (JiTT) and Peer Instruction. This paper presents a one-year retrospective on this process, with emphasis on lessons learned, the impact on student learning and satisfaction, and next steps. Student response to the change has been uniformly enthusiastic. The actual FCI gain achieved in this first year was G=0.31, a modest advance over the 0.25 reported for traditionally taught courses. Spurred by this early success, implementation of JiTT has spread to courses in Biology and Mathematics. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D29.00008: A Tale of Two Curricula: The performance of two thousand students in introductory electromagnetism Michael Schatz, Matthew Kohlmyer, Marcos Caballero, Ruth Chabay, Bruce Sherwood, Richard Catrambone, Marcus Marr, Mark Haugen, Lin Ding Student performance in introductory calculus-based electromagnetism (E\&M) courses at four large research universities was measured using the Brief Electricity and Magnetism Assessment (BEMA). Two different curricula were used at these universities: a traditional E\&M curriculum and the Matter \& Interactions (M\&I) curriculum. At each university, post-instruction BEMA test averages were significantly higher for the M\&I curriculum than for the traditional curriculum. The differences in post-test averages cannot be explained by differences in variables such as pre-instruction BEMA scores, grade point average, or SAT scores. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D29.00009: Improving clicker questions for enhanced learning in the interactive physics classroom Ertan Salik Classroom response systems, or clickers, have become widely used in physics classes in the last decade. Physics education research has demonstrated clearly that it is not the clicker as an electronic tool, but the interactive learning that occurs through clickers is what improves student learning. For clickers to work as expected, however, many subtle details need to be addressed. An instructor can start using many questions developed for the purpose of peer instruction. For a particular student population, and for a particular learning environment and constraints of the educational institution, questions used and the instructor's teaching style may need to be altered over time. We will present a systematic way of improving clicker questions and one's own teaching style utilizing data collected during clicker sessions. In addition, by adding a small writing component to some clicker questions, one can simply peek into student reasoning in order to determine preconceptions and misconceptions. Such direct knowledge of student reasoning in one's own class may be very revealing, and help improve learning ultimately. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D29.00010: Propagations of the AAPT New Faculty Workshop: A case study of the infusion of student-centered technological and pedagogical innovations in the introductory physics program at West Point Bryndol Sones Since 2002, the Department of Physics at West Point has been the fortunate recipient of yearly attendance at the AAPT New Faculty Workshop. This sustained involvement has contributed directly to enhancements in our two-semester introductory physics program. Two aspects of West Point's environment make our involvement with the workshop especially fruitful: our diverse students and our frequent faculty turn-over. We teach to over 1100 students with majors across the entire spectrum. The majority of our faculty is an active duty Army officer here for just three years. At West Point, we rely on the workshop as a wellspring for faculty development, technological innovation, and pedagogical refinement. In the past few years, we have incorporated aspects of peer instruction, activity-based learning, and tutorials for student discovery. On the technological side, we now have TabletPCs for faculty, rf response cards (TurningPoint), high speed video analysis (LoggerPro) projects, and video tutoring capabilities (Camtashia). Student achievement is measured through our traditional course evaluation tools as well as nationally recognized standardize tests. Results will are discussed in the presentation. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D29.00011: Instilling best educational practices into future physics professionals and faculty Philip G. Collins A primary aim of the New Faculty Workshop (NFW) has been to communicate best educational practices in faculty beginning their teaching careers. However, further amplification of NFW goals is achieved by providing similar content and training to Ph.D. candidates working as Teaching Assistants (TAs). NFW experience led to the successful creation at UCI of a relatively extensive, 30-hour training course now required of every graduate student in the Dept. of Physics and Astronomy. Half of the training occurs before the first week of classes, and focuses on peer instruction, active learning, and results from Physics Education Research. This orientation segues into peer evaluation as first-time TAs and soon-to-be TAs practice teaching styles for each other and evaluate videos of each other teaching their actual courses. This course directly trains 25-30 graduate students each year, indirectly affecting dozens of discussion sections and the experience of nearly 2000 students per quarter. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D29.00012: Applying New Faculty Workshop Lessons Learned to Intermediate-Level Physics Courses Melissa Eblen-Zayas Although much innovative work on physics teaching techniques and curriculum has been carried out at the level introductory physics, there has been less focus on the intermediate-level physics courses. Since my participation in the new faculty workshop, I have been particularly interested in finding ways to implement peer instruction and other active learning strategies in sophomore and junior level physics classes. Many curricular resources are available for introductory physics, but finding appropriate materials for courses beyond the introductory level is more challenging. I will discuss the use of comPADRE resources as well efforts to develop original activities to promote active learning and conceptual understanding, and I will highlight the successes and challenges of integrating these types of activities in intermediate-level physics courses. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D29.00013: From Syllabus To Diagnostic Exam: Learnings from the New Faculty Workshop Applied In the Intro Physics Classroom Michael Tanoff Kalamazoo College offers a ``one-size-fits-all'' concept-based introductory physics sequence. The widely varying demographic composition of the class --- including majors in biology, chemistry, pre-med, physics, and math, along with occasional humanities majors --- adds obvious challenges to the successful learning experience. As such, educational techniques that apply across the demographic are required. Several ideas presented at the Fall 2005 New Faculty Workshop apply to the needs of this broad range of students at Kalamazoo College, including an ``organic'' course syllabus that has been allowed to grow to whatever extent necessary to address recurring student concerns and misunderstandings about course expectations, policies, and guidelines, and to provide advice on recurring themes; peer instruction for maximizing classroom value; and hiring teaching assistants with first hand experience in the course and the labs. Details on implementing these techniques, including developing a syllabus with unusual section headings such as ``Attendance and Homework Dramas'' and ``Introductory Physics Survival Requirements,'' will be presented. Success of the techniques, as evidenced by performance on diagnostic exams, class attendance, and comments from course evaluations, will be discussed. [Preview Abstract] |
Session D30: Focus Session: Cobaltite and Manganite films
Sponsoring Units: DMP GMAGChair: Chris Leighton, University of Minnesota
Room: 334
Monday, March 16, 2009 2:30PM - 3:06PM |
D30.00001: Emergent Phenomena in Spatially Confined Manganites Invited Speaker: There is evidence that chemically disordered single crystal manganites exhibit electronic inhomogeneity in which areas with vastly different electronic and magnetic properties form and coexist in phase separated domains ranging in size from a few nanometers to micrometers. This phase separation is of particular interest, as it has been suggested that it is the central feature that leads to colossal magnetoresistance in manganites, the Mott transition in VO2 and may play a part in high-TC superconductivity in cuprates. We will discuss our ongoing efforts to answer fundamental questions about the specific role of PS in complex oxides using a novel spatial confinement technique. Unlike transport measurements on bulk or thin films where the electrons follow only the metallic path of least resistance, spatially confining a phase separated material to the scale of its inherent domains forces electrons to travel through both the metallic and insulating regions that lie along the conduction path. This has led to observations of several new phenomena such as a reemergent metal-insulator transition, ultra-sharp jumps in resistivity at the metal-insulator transition, and the first high resolution observation of stable single domain electronic phase transitions in time. La(5/8-x)Pr(x)Ca(3/8)MnO(3) (LPCMO) is used as a model system due to its large scale electronic phase separation into ferromagnetic metal (FMM) and charge ordered insulator (COI) electronic phase domains and well documented spin-charge-lattice interactions. These properties allow us to isolate domains through conventional wet etch techniques and offer a wide range of tunability through doping and substrate strain. This ability to control key elements of the underlying complexity and observe the resulting changes in the emergent behavior help answer questions about the fundamental physics that rule complex materials. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D30.00002: Electric field induced anisotropic transport properties of phase separated (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films Hyoung Jeen Jeen, Alessandra Gallastegui, Amlan Biswas The perovskite manganese oxide (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) exhibits electronic phase separation i.e., a ferromagnetic metallic phase and a charge ordered insulating phase coexist in a certain temperature range. It was shown that in such a phase separated state thin films of manganites show a colossal electroresistance (CER) although the mechanism driving this phenomenon is still unknown. We present transport measurements which show that the CER is due to an electric field driven anisotropy in phase separated manganites. LPCMO thin films were grown on NdGaO$_{3}$ (110) substrates using Pulsed Laser Deposition. A cross shaped micro-structure, with 60 by 10 $\mu $m legs, was fabricated using UV photolithography and chemical etching. We observe CER close to the insulator to metal transition temperature (T$_{IM})$ in the longitudinal direction i.e. parallel to the applied electric field. We simultaneously measure the transverse resistance in the other (orthogonal) leg of the microstructure. We observe a clear anisotropy in the conduction of the cross shaped microstructure which could be the origin of CER in manganites. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D30.00003: Colossal electroresistance in phase separated manganite nanobridges G. Singh-Bhalla, A. Biswas, A. F. Hebard We have examined the electric field effect on the nanometer scale in the manganite (La,Pr,Ca)MnO{\$}{\_}{\{}3{\}}{\$}which is well known for its micrometer scale phase separation (PS) into coexisting metallic and insulating regions. When thin films of this material are patterned into micrometer and nanometer wide bridges, alternating insulating and metallic regions may form along the length of the bridge within the PS temperature range. At the onset of PS, nanoscale ferromagnetic islands appear within the insulating antiferromagnetic background. Within this temperature range, transport properties along the length of the bridge are reminiscent of transport across metallic islands in the Coulomb blockade regime. Applications of a magnetic field induce unusual bifurcations in the current-voltage characteristics implying either a change in resistance arising from spin canting or a change in the ferromagnetic phase fraction. Next, within the micrometer scale PS temperature regime, current-voltage measurements reveal colossal, step like drops in resistance with increasing current. We will discuss our results both in the context of previously considered models for manganite electroresistance and new interpretations with a focus on the microscopic details of the metallic and insulating regions. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D30.00004: Phase modification of La$_{1/4}$Pr$_{3/8}$Ca$_{3/8}$MnO$_{3}$ thin films by light, magnetic field and applied stress Justin Olamit, Mikhail Zhernenkov, Mike Fitzsimmons, Haile Ambaye, Valeria Lauter, Hyoung Jeen Jeen, Amlan Biswas Complex oxide materials exhibit a wide variety of fascinating electromagnetic properties related to the coexistence of multiple electronic and magnetic phases.[1,2] The temperature-magnetic field phase diagram of La$_{0.27}$Pr$_{0.40}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) is intriguing; a ferromagnetic metallic (FMM) phase, charge ordered insulating (COI) phase and conditions where both phases coexist are accessible with changes in temperature and magnetic field.[3] We have performed neutron reflectometry to understand the emergence of the FMM phase in a COI matrix of a LPCMO thin film as functions of temperature, irradiation with light, magnetic field and stress. Specular reflectivity reveals the emergence of ferromagnetism below the phase transition temperature. Diffuse scattering shows that the length scale of FM domains is 1-2 microns. [1] Ch. Renner et al., Nature \textbf{416}, 518 (2002). [2] L. Zhang et al., Science \textbf{298}, 805 (2002). [3] T. Dhakal et al., Phys. Rev. B \textbf{75}, 092404 (2007). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D30.00005: Real-Space Imaging of Electronic Phase Separation in a Mn-Doped Bilayered Ruthenate Tae-Hwan Kim, M. Angst, R. Jin, X.G. Zhang, J.F. Wendelken, A.P. Li, B. Hu, E.W. Plummer Transition-metal oxides with multiple nearly degenerate states show very complicated phase diagrams. Small perturbations can often dramatically change their functionalities. It is believed that electronic phase separations (PS) play an important role in the exotic functionality. Direct experimental observation of PS has thus become crucial to understanding underlying mechanisms of the striking functionalities. We have studied the PS and the evolutions of phase domains with temperature near the Mott transition in a Mn-doped bilayered ruthenate Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$. Our experimental approach combines electron microscopy, scanning tunneling microscopy, and electron transport spectroscopy, which provide unprecedented capabilities of imaging PS and interrogating individual microscopic domains in situ. A quantitative correlation has been determined between the macroscopic metal-insulator transition and the microscopic phase domain percolation in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D30.00006: Mapping the phase boundaries in thin-film manganites using scale-invariant dielectric response P. Mickel, G. Singh-Bhalla, S. Tongay, A. Biswas, A. F. Hebard Magnetocapacitance techniques[1] have been used in a study of (La$_{1-y}$Pr$_{y})_{1-x}$Ca$_{x}$MnO$_{3}$ (LPCMO) thin films to determine the range of phase space, described by frequency ($\omega )$, temperature ($T)$ and field ($H)$, over which a dielectric response of the form, C"($\omega $,T,H) = [C'($\omega $,T,H) - C$_{\infty }$]$^{\gamma }$ , is found to hold. This power-law scaling collapse (PLSC) of the complex capacitance (C', C"), expressed in a Cole-Cole formulation, differs from the well-known ``universal'' dielectric response (UDR) [2], where the exponent $\gamma $ = 1. The influence of film thickness and stoichiometry on the extent of the PLSC region is investigated with the implementation of a new phase-space mapping technique. The mappings clearly illustrate the onset of phase competition in LPCMO, delineating boundaries which correspond to capacitive minima at low temperatures, where the first-order insulator-metal transition occurs, and to the second-order paramagnetic-insulator/charge-ordered-insulator transition at higher temperatures, where a resistive transport signature exists in bulk but not in thin films. Modeling with distributions of UDR elements corresponding to the different manganite phases gives a good qualitative account of the observed behavior, and can lead to the determination of individual phase fractions [1] R. Rairigh, Nature Physics 3, 551 - 555 (2007) [2] Jonscher - J. Phys. D: Appl. Phys. 32* *(1999) [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D30.00007: Direct measurement of the low-temperature spin-state transition in epitaxially strained LaCoO$_{3}$ thin films Robert Klie, Guang Yang, Yuan Zhao The perovskite oxide LaCoO$_{3}$ exhibits an anomaly in its magnetic susceptibility at 80 K associated with a thermally excited transition of the Co$^{3+}$-ion spin. We will show that atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy in combination with ab-initio first-principles DFT calculations can be utilized to measure the spin-state transition in LaCoO$_{3}$. In particular, we utilize in-situ cooling experiments in a transmission electron microscope to demonstrate that the O K-edge pre-peak is sensitive to the Co$^{3+}$-ion spin-state. Our experimental results will be compared to first-principles calculations, and we will conclude that the thermally excited spin-state transition occurs from a low to an intermediate spin state, which can be distinguished from the high-spin state. Next, we will examine the effects of bi-axial strain and point defects in LaCoO$_{3}$ thin-films on the Co$^{3+}$-ion spin-state. We will show that a single-crystal pseudo-cubic LaCoO$_{3}$ (001) film can be successfully grown on LaAlO$_{3}$ (001). Moreover, we will show that the epitaxially strained LaCoO$_{3}$ film exhibits a ferro-magnetic transition at low temperature that was not observed in bulk LaCoO$_{3}$. We will discuss the origin of this transition and the possibility of stabilizing different Co$^{3+}$-ion spin-states in LaCoO$_{3}$ using interfacial strain. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D30.00008: First-principles study for low-spin LaCoO$_{3}$ with structurally consistent Hubbard $U$ Han Hsu, Koichiro Umemoto, Matteo Cococcioni, Renata Wentzcovitch We use the local density approximation + Hubbard $U$ (LDA+$U$) method to calculate the structural and electronic properties of low-spin LaCoO$_{3}$. The Hubbard $U$ is obtained by first principles and consistent with each fully-optimized atomic structure at different pressures. With structurally consistent $U$, the fully-optimized atomic structure agrees with experimental data better than the calculations with fixed or vanishing $U$. A discussion on how the Hubbard $U$ affects the electronic and atomic structure of LaCoO$_{3}$ is also given. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D30.00009: Direct observation of local magnetic properties in strain engineered lanthanum cobaltate thin films S. Park, Weida Wu, J. W. Freeland, J. X. Ma, J. Shi Strain engineered thin film devices with emergent properties have significant impacts on both technical application and material science. We studied strain-induced modification of magnetic properties (Co spin state) in epitaxially grown lanthanum cobaltate (LaCoO$_3$) thin films with a variable temperature magnetic force microscopy (VT-MFM). The real space observation confirms long range magnetic ordering on a tensile-strained film and non-magnetic low-spin configuration on a low-strained film at low temperature. Detailed study of local magnetic properties of these films under various external magnetic fields will be discussed. Our results also demonstrate that VT-MFM is a very sensitive tool to detect the nanoscale strain induced magnetic defects. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D30.00010: Understanding the Origin of Ferromagnetism in Strained LaCoO$_3$ Thin Films J.X. Ma, J. Shi, J.W. Freeland Using strain to control the behavior of strongly correlated materials offers new opportunities to control fundamental properties. For the case of magnetism, LaCoO$_3$ offers the ability to use strain through thin film growth to manipulate directly the spin-state of Co in this system. Here we present the results of a detailed polarized x-ray spectroscopy study of LaCoO$_3$ thin films grown on SrTiO$_3$(001) and LaAlO$_3$ (001) substrates. X-ray diffraction from 25 nm thin films confirm the films are fully strained in both cases and, for films under tensile strain, total moment magnetometry shows a clear transition to ferromagnetic state at $\sim$80K. X-ray absorption shows that the films grown from a LaCoO$_3$ target are slightly hole doped due to non-stoichiometry generated during growth (effective doping $\sim$ 0.1 holes per unit cell), which in the bulk is sufficient to destroy the low-spin state. However, even though the films are slightly hole doped, the films under tensile strain show long range ferromagnetic order unlike the bulk system. Since the films are insulating, these results are consistent with a ferromagnetic insulating state arising due to superexchange. Work at UCR is supported by ONR/DMEA under award H94003-08-2-0803. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D30.00011: Structure, Magnetism, and Transport in SrTiO$_{3}$(001) / La$_{1-x}$Sr$_{x}$CoO$_{3}$: Evidence for Interfacial Magnetic Phase Separation M.A. Torija, M. Sharma, C. He, J. Gazquez, M. Varela, M. Laver, B.B. Maranville, J.A. Borchers, C. Leighton Doped cobaltites have proven to be excellent choices for the study of the magneto-electronic phase separation phenomenon. Strong motivation exists for the study of these materials in films and heterostructures, the effect of dimensional confinement on this phase separation being a prime example. We investigated the structure, magnetism, and magnetotransport, in epitaxial La$_{1-x}$Sr$_{x}$CoO$_{3}$ on SrTiO$_{3}$ (001). We have observed deterioration in ferromagnetism and conductivity in the thin film limit (e.g. $<$ 8 nm at x = 0.50). We demonstrate that this can be definitively ascribed to interfacial magnetoelectronic phase separation. Key observations are the existence of an intercluster ``GMR'', anomalous multiterminal transport, strongly non-gaussian resistance fluctuations, and direct measurement of short-range ferromagnetic order by SANS. The thickness of the phase-separated region diverges as the doping is reduced from x = 0.50 to x = 0.18, and it can also be induced by deposition of SrTiO$_{3}$ overlayers. STEM/EELS data rule out the possibility of chemical phase separation proving that the deterioration in magnetic and electronic properties near the interface with SrTiO$_{3}$ is due to an intrinsic magnetic phase separation effect. [Supported by NSF and DOE]. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D30.00012: Electronic structure of La$_{1-x}$Sr$_{x}$CoO$_{3-\delta }$ in the presence of ordered oxygen vacancies Jaume Gazquez, M. Varela, M.P. Oxley, W. Luo, S.T. Pantelides, M.A. Torija, M. Sharma, C. Leighton, S.J. Pennycook Here we present a study of oxygen vacancy ordering in La$_{1-x}$Sr$_{x}$CoO$_{3-\delta }$ (LSCO) thin films using a combination of atomic resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS). Substituting Sr$^{2+}$ for La$^{3+}$ in LaCoO$_{3}$ results in either the formation of oxygen vacancies or an increase in the mean cobalt valence in order to preserve charge neutrality. At large concentrations, oxygen vacancies in LSCO form ordered structures with orientations determined by epitaxial strain. This talk will show how different O $K$-edge fine structures can be observed in EEL spectra obtained from different sites of the superstructure, while the Co $L$-edges are unchanged. These results, together with density functional theory and dynamical scattering calculations, suggest that there is no charge ordering but a modulation of the hole doping in these systems. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D30.00013: Magnetic phase separation-induced coercivity enhancement in epitaxial Nd$_{0.5}$Sr$_{0.5}$CoO$_{3}$ films M. Sharma, J. Gazquez, M. Varela, C. Leighton Interfacial magneto-electronic phase separation has recently been observed in epitaxial thin films of the doped perovskite cobaltite La$_{1-x}$Sr$_{x}$CoO$_{3}$ at doping values where no such phase separation exists in bulk. Such systems also display anomalously large coercivity, which is not understood. To achieve a better understanding of this phenomenon we have extended this study to Nd$_{1-x}$Sr$_{x}$CoO$_{3}$ (x = 0.5), the perovskite cobaltite with the largest coercivity in bulk. Thin films of Nd$_{0.5}$Sr$_{0.5}$CoO$_{3}$ are grown via high pressure reactive sputtering on SrTiO$_{3}$ (001) substrates. We have observed a rapid deterioration in magnetization and onset of large intercluster-type magnetoresistance below a critical thickness of 80 {\AA}, signatures of interfacial magneto-electronic phase separation also seen in our earlier work on La$_{1-x}$Sr$_{x}$CoO$_{3}$. The temperature, angular, and thickness dependence of the coercivity ($H_{c})$ was studied using magnetoresistance. Low temperature $H_{C}$ values become very large (up to 3.6 Tesla) at low thickness, and a strong, superlinear $T$ dependence emerges. We propose that the coercivity enhancement arises due to efficient domain wall pinning by the inhomogeneous magnetically phase separated region near the SrTiO$_{3}$ substrate. [Preview Abstract] |
Session D31: Focus Session: Quantum Magnets
Sponsoring Units: GMAGChair: Ray Osborn, Argonne National Laboratory
Room: 335
Monday, March 16, 2009 2:30PM - 2:42PM |
D31.00001: Spin Relaxation in Pure and Doped GGG Michael Schmidt, Thomas Rosenbaum, Daniel Silevitch, Gabriel Aeppli, Sayantani Ghosh, Y.K. Verma Geometric frustration in Gadolinium Gallium Garnet (GGG) leads to local regions of correlated spins that can be manipulated without affecting the background spin susceptibility. These ``quantum protectorates'' can be accessed via the non-linear response at milliKelvin temperatures using a hole burning technique. We study the effect of impurities on both the structure of the spin clusters and the dissipation spectrum in Neodymium-doped GGG crystals via pump-probe ac magnetic susceptibility and direct optical measurements. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D31.00002: Bose-Einstein Condensation of Triplons in Ba$_3$Cr$_2$O$_8$ A.A. Aczel, Y. Kohama, M. Jaime, L. Balicas, K. Ninios, H.B. Chan, H.A. Dabkowska, G.M. Luke By performing heat capacity, magnetocaloric effect, torque magnetometry and force magnetometry measurements up to 33 T, we have mapped out the T-H phase diagram of the S = 1/2 spin dimer compound Ba$_3$Cr$_2$O$_8$. We found evidence for field-induced magnetic order between Hc1 = 12.52(2) T and Hc2 $\sim$ 23.6 T, with the maximum transition temperature Tc $\sim$ 2.7 K at H $\sim$ 18 T. There are many qualitative features of the data suggesting that the transition at Hc1 corresponds to a Bose-Einstein condensation of triplons universality class. These include the apparent preservation of U(1) symmetry for applied fields below Hc1, a highly symmetric phase diagram, and an absence of any magnetization plateaus in the magnetic torque and force measurements. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D31.00003: Specific heat measurements in the novel frustrated quantum magnets SrHo$_{2}$O$_{4}$ and SrDy$_{2}$O$_{4}$ A. D. Bianchi, B. Prevost, N. Kurita, F. Ronning, R. Movshovich, T. W. Klimczuk, M. Kenzelmann, R. J. Cava We investigated the specific heat of the novel geometrically frustrated quantum magnets SrHo$_{2}$O$_{4}$ and SrDy$_{2}$O$_{4}$ to determine the nature of their ground states. We present a study of the magnetic field dependence of specific heat $C_p(T,H)$ measured in a dilution refrigerator between 0.1 K and 4 K and a PPMS between 2 and 50~K for magnetic fields $H$ between 0 and 9~T. We subtracted the phonon background $C_{\mathrm{ph}}$ by using a temperature dependent Debye temperature determined from measurements on the non-magnetic structural analogue SrLu$_2$O$_4$. After this subtraction, in SrHo$_2$O$_4$ we observed a broad anomaly in the magnetic specific heat $C_{\mathrm{mag}} = C_p - C_{\mathrm{ph}}$ centered at 0.5 K in zero field. At high fields, we found a broad peak centered at 0.35 K which decreases with rising magnetic field. SrDy$_2$O$_4$ in zero field has a broad anomaly at 1.2~K. The peak broadens with increasing $H$ and its amplitude decreases, and by 5~T it is completely suppressed. By 50~K, each ion in the Dy compound has recovered 21.5~J/mol~K of its spin entropy, which is comparable to the entire spin entropy of a free Dy ion of $R \cdot ln(2J+1)$, whereas we observe only 11.1~J/mol~K for SrHo$_2$O$_4$. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D31.00004: The Interplay of Quantum Criticality and Frustration in Columbite Ribhu Kaul, SungBin Lee, Leon Balents CoNb$_2$O$_6$ is a remarkable material. It can be modeled as a lattice of Ising chains coupled to each other in a frustrated anisotropic triangular lattice in the basal plane perpendicular to the chain direction. Applying a strong transverse field tunes the chains through a quantum phase transition into a paramagnetic phase. The interplay between two of the most interesting features of correlated quantum physics, quantum criticality and geometric frustration, produces a rich phase diagram which reflects the fundamental underlying quantum many-body physics. Using a variety of analytic and numerical techniques, we map out the phase diagram of this material in both transverse and longitudinal fields and provide a comparison with experiment. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D31.00005: Quantum phase transitions of the asymmetric three-leg spin tube Toru Sakai, Masahiro Sato, Kouichi Okunishi, Yuichi Otsuka, Kiyomi Okamoto, Chigak Itoi We investigate quantum phase transitions of the S=1/2 three-leg antiferromagnetic spin tube with asymmetric inter-chain (rung) exchange interactions. On the basis of the electron tube system, we propose a useful effective theory to give the global phase diagram of the asymmetric spin tube. In addition, using other effective theories we raise the reliability of the phase diagram. The density-matrix renormalization-group and the numerical diagonalization analyses show that the finite spin gap appears in a narrow region around the rung-symmetric line, in contrast to a recent paper by Nishimoto and Arikawa [1]. The numerical calculations indicate that this global phase diagram obtained by use of the effective theories is qualitatively correct. In the gapless phase on the phase diagram, the numerical data are fitted by a finite-size scaling in the $c=1$ conformal field theory. We argue that all the phase transitions between the gapful and gapless phases belong to the Berezinskii-Kosterlitz-Thouless universality class [2]. \\[0pt] [1] S. Nishimoto and M. Arikawa, Phys. Rev. B 78 (2008) 054421.\\[0pt] [2] T. Sakai, M. Sato, K. Okunishi, Y. Otsuka, K. Okamoto and C. Itoi, Phys. Rev. B 78 (2008) 184415. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D31.00006: Critical behavior study of antiferromagnetism in isostructural La$_{2}$CuO$_{4+\delta }$ and La$_{2}$NiO$_{4+\delta }$ Benjamin White, John Neumeier, A. Erb Neutron diffraction [1] and nuclear quadruple resonance [2] experiments coupled with theory calculations provide substantial evidence to support the widely-accepted belief that the two-dimensional Heisenberg model describes the antiferromagnetic interactions in La$_{2}$CuO$_{4}$ (S = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ and La$_{2}$NiO$_{4}$ (S = 1). The heat capacity critical exponent $\alpha $, which could provide further evidence, has never been studied in these two compounds because of the weak nature of the anomalies at T$_{N}$. [3] We will present heat capacity and high-resolution thermal expansion measurements of La$_{2}$CuO$_{4}$ and La$_{2}$NiO$_{4}$ single crystals, grown by the floating-zone method, and an analysis of $\alpha $ within the context of predicted values for a variety of universality classes. This material is based upon work supported by the NSF (DMR-0504769) and US DOE Office of Basic Energy Sciences (DE-FG-06ER46269) [1] Y. Endoh et al., PRB 37, 7443 (1988); G. Aeppli and D.J. Buttrey, PRL 61, 203 (1988). [2] P. Carretta, A. Rigamonti, and R. Sala, PRB 55, 3734 (1997). [3] T. Kyomen et al., PRB 60, 14841 (1999); K. Sun et al., PRB 43, 239 (1991). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D31.00007: Physics of the spin gap in the $S=1/2$ Heisenberg antiferromagnet on kagome Invited Speaker: A combination of low spin and strong frustration makes the $S=1/2$ Heisenberg antiferromagnet on kagome a likely candidate for an unusual ground state and elementary excitations. Exact-diagonalization studies [1] on finite clusters point to a lack of magnetic order in the ground state and to an energy gap of order $J/20$ for $S=1$ excitations. The exact nature of the ground state and elementary excitations remains a subject of vigorous debate. Among the proposed ground states are chiral [2] and non-chiral [3] spin liquids and a valence-bond crystal (VBC) [4-5]; spin excitations range from deconfined spinons with a Bose [6] or Fermi statistics [2-3] to magnons [7]. We show that the system behaves as a collection of spinons, quasiparticles with $S=1/2$ and Fermi statistics, whose motion disturbs valence-bond order. Attraction between spinons, mediated by exchange, binds them into small, massive pairs of $S=0$ with a binding energy of $0.06 J$ [8]. The pair formation strongly suppresses the motion of individual spinons and makes the survival of the Singh-Huse VBC plausible. A spin excitation amounts to breaking up a pair into two (nearly) free spinons with $S=1$. The survival of the VBC is expected to lead to spinon confinement; however, small energy differences between various valence-bond configurations would make the confinement length large. \\[4pt] [1] Ch. Waldtmann et al., Eur. Phys. J. B \textbf{2,} 510 (1998).\\[0pt] [2] J. B. Marston and C. Zeng, J. Appl. Phys. \textbf{69,} 5962 (1991).\\[0pt] [3] M. B. Hastings, Phys. Rev. B \textbf{63,} 014413 (2000).\\[0pt] [4] P. Nikolic and T. Senthil, Phys. Rev. B \textbf{68,} 214415 (2003).\\[0pt] [5] R. R. P. Singh and D. A. Huse, Phys. Rev. B \textbf{76,} 180407 (2007).\\[0pt] [6] S. Sachdev, Phys. Rev. B \textbf{45,} 12377 (1992).\\[0pt] [7] R. R. P. Singh and D. A. Huse, arXiv:0801.2735. \\[0pt] [8] Z. Hao and O. Tchernyshyov, the subsequent talk. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D31.00008: Bound state of two spinons in a S=1/2 Heisenberg antiferromagnet on kagome Zhihao Hao, Oleg Tchernyshyov Elser et al. [1,2] identified a promising route to the ground state of the S=1/2 Heisenberg antiferromagnet on kagome via dimerized states, in which 3/4 triangles contain a valence bond. Quantum dynamics arises from the remaing ``defect'' triangles lacking a valence bond. We study an isolated defect on the Husimi cactus, a tree-like modification of kagome [1,3]. We show that the defect can be viewed as a bound state of two fermionic spinons with S=0. The bound state is small, on the order of 1.5 lattice spacings. It is localized and has a binding energy of 0.06 J relative to the 2-spinon continuum. No bound state is formed by 2 spinons with S=1. We argue that the pair-binding energy determines the spin gap of the kagome antiferromagnet. Our result for the gap agrees with the existing numerics [4,5]. [1] V. Elser and C. Zeng, Phys. Rev. B 48, 13647 (1993). [2] C. Zeng and V. Elser, Phys. Rev. B 51, 8318 (1995). [3] P. Chandra and B. Doucot, J. Phys. A: Math. Gen. 27, 1541 (1994). [4] Ch. Waldtmann et al., Eur. Phys. J. B 2, 510 (1998). [5] R. R. P. Singh and D. A. Huse, arXiv:0801.2735. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D31.00009: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:42PM - 4:54PM |
D31.00010: Order and Disorder in AKLT Antiferromagnets in Three Dimensions Siddharth Parameswaran, S.L. Sondhi, Daniel Arovas The models constructed by Affleck, Kennedy, Lieb, and Tasaki (PRL {\bf 59}, 799 (1987)) describe a family of quantum antiferromagnets on arbitrary lattices, where the local spin $S$ is an integer multiple $M$ of half the lattice coordination number. The equal time quantum correlations in their ground states may be computed as finite temperature correlations of a classical $\textsf{O}(3)$ model on the same lattice, where the temperature is given by $T=1/M$. In dimensions $d=1$ and $d=2$ this mapping implies that all AKLT states are quantum disordered. We consider the $d=3$ case where the nature of the AKLT states is now a question of detail depending upon the choice of lattice and spin; for sufficiently large $S$ some form of N{\'e}el order is almost inevitable. On the unfrustrated cubic lattice, we find that all AKLT states are ordered while for the unfrustrated diamond lattice the minimal $S=2$ state is disordered while all other states are ordered. On the frustrated pyrochlore lattice, we find (conservatively) that several states starting with the minimal $S=3$ state are disordered. These are a significant addition to the catalog of magnetic Hamiltonians in $d=3$ with ground states known to lack order on account of strong quantum fluctuations. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D31.00011: Global phase diagrams of frustrated quantum antiferromagnets in two dimensions: doubled Chern-Simons theory Cenke Xu, Subir Sachdev We present a general approach to understanding the quantum phases and phase transitions of quantum antiferromagnets in two spatial dimensions. We begin with the simplest spin liquid state, the Z$_2$ spin liquid, whose elementary excitations are spinons and visons, carrying Z$_2$ electric and magnetic charges respectively. Their dynamics are expressed in terms of a doubled U(1) Chern-Simons theory, which correctly captures the ``topological'' order of the Z$_2$ spin liquid state. We show that the same theory also yields a description of the variety of ordered phases obtained when one or more of the elementary excitations condense. Field theories for the transitions and multicritical points between these phases are obtained. We also survey experimental results on antiferromagnets on the anisotropic triangular lattice, and make connections between their phase diagrams and our results. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D31.00012: Stability of the U(1) spin liquid with spinon Fermi surface in 2+1 dimensions Sung-Sik Lee We study non-perturbative stability of a 2+1 dimensional critical spin liquid state, the U(1) spin liquid with a spinon Fermi surface. By mapping the spinon Fermi surface into an infinite set of 1+1 dimensional chiral fermions, we show that an instanton has an infinite scaling dimension for any nonzero N, where N is the number of spinon flavors. Therefore, the spin liquid state can be stable against confinement in physical systems, such as spin 1/2 magnets on the triangular lattice. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D31.00013: Finite-size scaling of string order parameters characterizing the Haldane phase Hiroshi Ueda, Hiroki Nakano, Koichi Kusakabe We have developed a numerical procedure to clarify the critical behavior near a quantum phase transition by analyzing a multi- point correlation function characterizing the ground state. The procedure focuses the gradient of the inversed-system-size dependence of the correlation function on a logarithmic plot. It requires only the correlation functions of several finite sizes under the same condition as a candidate for the long-range order. We apply the analysis to the string order parameter of the $S=1$ $XXZ$ chain with uniaxial single-ion anisotropy obtained by the density matrix renormalization group method. The present analysis gives precise estimates of transition points and critical exponents, $\nu$ and $\eta$, in Ising transitions, Gaussian transitions, and Berezinskii- Kosterlitz- Thouless transitions are consistent with results obtained from the analysis of the energy-level structure. This method will contributes much for a direct observation of quantum phase transitions. [Preview Abstract] |
Session D32: Focus Session: Spin Dependent Physics in Organic Materials and Graphene
Sponsoring Units: GMAG DMPChair: Tiffany Santos, Argonne National Laboratory
Room: 336
Monday, March 16, 2009 2:30PM - 2:42PM |
D32.00001: Magnetism by nonmagnetic defects in a 2D BN sheet: \textit{ab initio} studies Ru-Fen Liu, Ching Cheng This study attempts to resolve: 1) whether a long-range magnetic order can be established in a sp material, 2) whether the magnetic properties can be controlled through defects in the previous systems. Through studying different defects concentrations in a 2D BN sheet by the first-principles methods, we found that, despite most of the defects doped BN sheet only lead to formations of local moments, there are systems exhibiting long-range ordered magnetic moment with estimated T$_{C}\approx $70K[PRB 76, 014405 (2007)]. Our latest results demonstrate that a ferromagnetic free electron gas due to Stoner instability is possible to develop[RFL{\&}CC, to be published]. This result is contrary to the direct transition from paramagnetism to Wigner crystal in a 2D uniform electron gas established recently [N.D. Drummond{\&}R.J. Needs, to be published]. A contrastive study between this Stoner magnet, i.e. a defect system with N atoms replaced by O impurities (O$_{N})$, and a metallic ferromagnetic systems with localized moments (Si$_{B})$ throughout all the considered defect concentrations is studied by their band structure, partial DOS's and the defect-concentration-dependent magnetization energies. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D32.00002: Magnetoresistance in hydrogen-doped graphene nanoribbons. F. Munoz-Rojas, D. Soriano, J. Fernandez-Rossier, J. J. Palacios Recent works have focused on hydrogen-doped graphene, both in the diluted and highly doped concentrations. It is known that a single hydrogen atom on top of a carbon atom in graphene has a magnetic moment. In the case of a low concentration of hydrogen dopants, it is believed that the ground state features local moments with zero total spin. Application of a strong enough magnetic field can spin polarize the system, in analogy with diluted magnetic semiconductors. In this work we study whether this spin order changes the resistance of the system. We study the relation between conductance and spin order for hydrogen-doped graphene armchair nanoribbons. We use both mean field Hubbard model and density functional theory calculations and compare results from both approaches. For the latter one, B3LYP hybrid functional is used. The conductance is calculated for the diluted limit. We use the Landauer formalism with the Green's Function Approach for the conductance calculation. We find that the conductance in these systems is significantly affected by spin order. Thus, we predict magnetoresistance in graphene ribbons doped with Hydrogen. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D32.00003: Spin dependent Transport in Thin Graphite and Few Layer Graphene Christopher Malec, Dragomir Davidovic Few layer graphene as well as thin graphite samples are measured by local and non-local spin injection techniques at 4.2 K. Both spin valve, and spin precession measurements are performed. Spins remain coherent over micron length scales. Latest results will be discussed. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D32.00004: Magnetism and magnetic interactions in graphene and graphite Invited Speaker: Magnetic materials and nanostructures based on carbon and other light elements provide a number of attractive opportunities for future information technologies such as spintronics and quantum information processing. In this talk, I review the first-principles studies of the magnetism induced by defects and edges in graphene and graphite. We show that in graphene the single-atom defects (e.g. vacancies and hydrogen chemisorption) induce the spin-polarized defect states [1,2]. The coupling between the magnetic moments is either ferromagnetic or antiferromagnetic, depending on whether the defects correspond to the same or to different sublattices of the graphene lattice, respectively. These results explain the recent experimental observations of high-temperature ferromagnetism in proton-irradiated graphite. Similarly, the zigzag edges of graphene are predicted to induce localized magnetic moments which can serve as a basis for novel spintronic devices. We address the question of the spin correlation length at finite temperatures in this one-dimensional magnetic system and establish the limitations of the proposed spintronic devices [3]. Finally, I consider the hyperfine interactions (i.e. the magnetic interactions between the spins of electrons and nuclei) in carbon nanostructures and materials [4]. Possible approaches for achieving long electron spin decoherence times in graphene-based nanostructures are discussed. \newline [1] O. V. Yazyev and L. Helm, Phys. Rev. B {\bf 75}, 125408 (2007). \newline [2] O. V. Yazyev, Phys. Rev. Lett. {\bf 101}, 037203 (2008).\newline [3] O. V. Yazyev and M. I. Katsnelson, Phys. Rev. Lett. {\bf 100}, 047209. \newline [4] O. V. Yazyev, Nano Lett. {\bf 8}, 1011 (2008). [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D32.00005: Determining the spatial and spin anisotropy of reduced-dimensionality Cu-based magnets using EPR, ultra-high-field magnetization and simulations Susan Cox, Ross McDonald, John Singleton, Pinaki Sengupta, Paul Goddard, Stephen Blundell, Janez Bonca, Samir El Sawish, Jamie Manson, John Schlueter Pulsed-field magnetization experiments (up to 85 T) and electron paramagnetic resonance (EPR) experiments ($10-110$~GHz) are reported on a family of organic Cu-based two-dimensional (2D) Heisenberg magnets. The low-$T$ $M(H)$ relationship is concave, with a sharp transition to a saturation value at a critical field $H_{\rm c}$. Monte-Carlo simulations including a finite interlayer exchange energy quantitatively reproduce the data. Thus, one can obtain accurate values for both intra- and interlayer exchange energies. The EPR spectra show pronounced changes in effective $g$ factor, linewidth and zero- field intercept at temperatures, fields and frequencies of the same energy scale as the dominant exchange parameter. The EPR results are modeled using finite-cluster- size methods, and the data are well matched by an easy-plane spin anisotropy in the range $0.01-0.05$. Thus, EPR measurements allow the spin orientation dependence of the exchange interaction to be determined. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D32.00006: Ferromagnetic transition coupled to magnetoelastic interactions J.L. Musfeldt, L.I. Vergara, T.V. Brinzari, L.C. Tung, Y.J. Wang, J.A. Schlueter, J.L. Manson We investigate the magneto-infrared response of CuHF$_2$(pyz)$_2$BF$_4$, a quasi-two-dimensional Heisenberg antiferromagnet, in order to probe the microscopic aspects of magnetoelastic coupling through the field-driven antiferromagnetic to ferromagnetic transition. The ferromagnetic transition is accompanied by substantial changes in the out-of-plane pyrazine ring distortion and bending modes giving rise to an overall softer lattice. The size of these field-induced distortions tracks the bulk magnetization demonstrating that the ferromagnetic transition is coupled to magnetoelastic interactions in this material. We discuss these results in terms of local structural distortions and the effect on in-plane superexchange interactions. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D32.00007: Magnetic phase diagram of a 2D quantum Heisenberg antiferromangetic compound Cu(pz)$_{2}$(ClO$_{4})_{2}$ Fan Xiao, Nat Fortune, Christopher Landee, Mark Turnbull Cu(pz)$_{2}$(ClO$_{4})_{2}$ is a 2D quantum Heisenberg antiferromagnet with an exchange strength of 17.5(5) K and a zero-field ordering temperature of 4.25 K. The ordering temperature has been found to be affected by an applied field. The phase diagram of Cu(pz)$_{2}$(ClO$_{4})_{2}$ is determined by measuring the magnetization and the in-field specific heat. The behavior of the ordering temperature can be interpreted as a field induced 2D Heisenberg to 2D XY crossover. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D32.00008: Gapped quantum spin-liquid state in a frustrated triangular magnet $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ Minoru Yamashita, Norihito Nakata, Yuichi Kasahara, Takahiko Sasaki, Naoki Yoneyama, Norio Kobayashi, Satoshi Fujimoto, Takasada Shibauchi, Yuji Matsuda Unveiling the nature of quantum-spin-liquids (QSL) states, quantum fluctuation-driven disordered ground states, has been a central challenge in condensed matter physics. Especially the nature of the low-lying spin excitations and the presence/absence of the ``spin gap'' have been of great interest. Recently, NMR measurements have shown that a QSL state is realized in $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ with a nearly isotropic 2D triangular lattice structure. Here we report on our thermal-transport measurements in this compound down to 80~mK. We find that the QSL state has a full gap of $\sim 0.5$~K ($\sim J/500$) and the gap is hardly affected by magnetic fields up to 10~T [1], which sharply contradict recent reports of heat capacity measurements reporting a finite $\gamma$-term. We will discuss some possibilities to explain the tiny spin gap in this triangular system. \\[0pt] [1] M. Yamashita et al., Nature Physics (in press). [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D32.00009: Neutron scattering measurements of spin excitations in the spin ladder compound (pip)$_2$CuBr$_4$* A.T. Savici, C.L. Broholm, G.E. Granroth, S.E. Nagler, K.P. Schmidt, G.S. Uhrig, D.M. Pajerowski, M.W. Meisel, D.R. Talham, C.M. Brown Recent theoretical and experimental work on S=1/2 ladders has
been inspired, in part, by the realization of a novel quantum
spin liquid state. Bulk magnetization measurements indicate that
(C$_5$H$_{12}$N)$_2$CuBr$_4$ (BPCB) is a two-leg spin ladder with
stronger coupling along the rung ($J_\bot$) than along the leg
direction ($J_\|$) [1]. Here we report neutron spectroscopy
measurements performed on a deuterated BPCB. We show that
$J_\bot$ and $J_\|$ are consistent with the previous
measurements. No dispersion in the inter-ladder direction means
that the ladders are magnetically isolated, likely due to
frustrated inter-ladder exchange. We show that any diagonal
exchange is $ |
Monday, March 16, 2009 4:42PM - 4:54PM |
D32.00010: Pressure-dependent $\nu$(CC) and $\nu$(CN) Raman modes of the molecule-based magnets M[TCNE](NCMe)$_2$X Alexander E. Midgley, Konstantin Pokhodnya, C. Olson, A.N. Caruso, Michael B. Kruger M[TCNE] (M=V, Fe, Mn, Co, Ni; TCNE=tetracyanoethylene) molecule-based magnets demonstrate high magnetic ordering due to a strong antiferromagnetic interaction between the unpaired d- and \mbox{p-electrons } of the metal ions and the ligands; however, the type of bonding involved in the superexchange mechanism remains unclear. The Raman active C=C vibration depends solely upon the degree of charge transfer from the metal ion to the $\pi$* antibonding orbital of the ligand, therefore the strength of the vibration is only sensitive to backbonding. Raman spectra of the M[TCNE](NCMe)$_2$X (M=Fe, Mn, Ni; X=FeCl$_4$, SbF$_6$) molecule-based magnets were collected in a diamond anvil cell at pressures up to 36 kbar. The observed pressure-induced strengthening of the $\nu$(CC) and $\nu$(CN) Raman modes provides a clearer picture of the type and degree of backbonding, which will ultimately help build a model of how superexchange is occurring in these systems. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D32.00011: Bonding, Backbonding and Spin-Polarized Molecular Orbitals: Basis for Magnetism and Semiconducting Transport in V[TCNE]$_{x\sim 2}$ Jeffrey Kortright, Derek Lincoln, Ruth Shima Edelstein, Arthur Epstein V[TCNE]$_{x\sim 2}$ films exhibit magnetic order up to 400 K, magneto-resistance, and photo-induced magnetism. Yet the spin-polarized interactions between the TM and molecular species underlying these properties have remained elusive, in part because of its structural disorder. Using element-specific x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V $L$ edges, and the C and N $K$ edges we have gained new insight into these mechanisms [1]. We find evidence for covalent bonding between the V $e_{g}$ and TCNE \textit{$\sigma $} MO states, and a weaker interaction between V $t_{2g}$ and TCNE \textit{$\pi $} MO states, consistent with a generalized bonding/backbonding model with V octahedrally coordinated by N in \textit{$\sigma $}-bridging positions between TCNE radical anions. C and N XAS and MCD reveal spin-polarized splitting of the former LUMO of neutral TCNE, indicating that a direct exchange interaction underlies these properties. This indicates an active role of TCNE$^{\bullet -}$ in the magnetic properties of extended V[TCNE]$_{x\sim 2}$ and related systems, which is distinctly different from superexchange models generally used to describe magnetic Prussian blue analogs. [1] Phys. Rev. Lett. \textbf{100}, 257204 (2008). [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D32.00012: XPS and UV/Vis MCD studies of M[TCNE] organic-based magnets Saad Janjua, Konstantin Pokhodnya, Marcus Driver, Anthony Caruso M[TCNE] (M = V, Fe, Mn, Ni; TCNE = tetracyanoethylene) organic-based magnets provide a systematic means of studying magnetic superexchange by varying the 3d t$_{2g}$ and e$_{g}$ filling. X-ray photoemission binding energy spectra of core electrons C (1s), N (1s) and M (3p) were used to study valency and bond type, giving a correlation between binding energy and transition temperature. UV/Vis Magnetic Circular Dischroism studies were conducted to investigate lowest unoccupied state and onsite Coulomb repulsion for both M 3d and TCNE $\pi $*. This talk will focus on providing empirical evidence of the near Fermi edge spin polarized electron structure in the context of magnetic exchange [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D32.00013: ABSTRACT WITHDRAWN |
Session D33: Superconducting Theory I
Sponsoring Units: DCMPChair: Dennis Newnes, IBM
Room: 403
Monday, March 16, 2009 2:30PM - 2:42PM |
D33.00001: Giant nonlinear electron-lattice interaction in cuprate superconductors, and origin of the pseudogap Dennis Newns The pseudogap is a key property of the cuprate superconductors, whose understanding should illuminate the pairing mechanism. Recent data support a close connection between the pseudogap and an oxygen-driven C4 symmetry breaking within the CuO$_{2}$ plane unit cell. Using \textit{ab initio} Molecular Dynamics, we show the existence of a strong nonlinear electron-oxygen vibrator coupling in two cuprates. In a mean field approach applied to this coupling, we derive a C4 splitting/pseudogap phase diagram in agreement with experiment - providing an explanation for the pseudogap phenomenon from first principles. The implications for superconductivity, Fermi surface arcing, and other properties are discussed. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D33.00002: An explanation of the dichotomy between Fermi arcs and Fermi pockets in underdoped high-$T_{c}$ superconductors Xun Jia, Sudip Chakravarty We have numerically computed the spectral function $A(\vec{k},\omega)$ of an underdoped cuprate superconductor for the $d$-density wave state subject to a long range correlated disorder. The intensity of the spectral function is significantly reduced for the electron pockets for an intermediate range of correlation length, but the Fermi arcs remain quite intact. This result provides one possible explanation as to why the electron pockets are not observed in angle resolved photoemission experiments. A calculation of Shubnikov-de Haas (SdH) oscillations using a real space transfer matrix method shows that two main frequencies are still present in the presence of a moderate amount of white noise disorder. The SdH oscillations in other relevant broken symmetry states are also computed. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D33.00003: Quantum oscillations in a highly renormalized Fermi liquid model of Fermi pockets in underdoped cuprates Tudor Stanescu, Victor Galitski, Dennis Drew Motivated by the recent experimental observation of quantum oscillations in the underdoped cuprates, which suggest the existence of small electron pockets characterized by a relatively large cyclotron mass, we address two basic questions: 1) How can one explain the relatively large cyclotron effective mass observed experimentally and its relation with the effective Hall mass? 2) Why the electron pockets are not seen in ARPES experiments? We propose an explanation based on a model of a highly renormalized Fermi liquid characterized by a reconstructed Fermi surface and strongly momentum-dependent quasiparticle properties. We find that the cyclotron mass is enhanced by a factor $< 1/Z >$, while the effective Hall mass is proportional to $< Z >/< Z^2 >$, where $<...>$ implies an averaging over the Fermi surface. If the Z-factor becomes small in some part of the Fermi surface, the cyclotron mass is enhanced sharply while the infrared Hall mass may remain small. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D33.00004: Superconductivity near structural phase transition: the case of NbN Simon Blackburn, Michel C\^ot\'e, Steven G. Louie, Marvin L. Cohen Using density functional theory within the local density approximation we report the study of the electron-phonon coupling in NbC$_{1- x}$N$_{x}$ crystals in the rocksalt structure. The Fermi surface of the system allows important nesting. The associated Kohn's anomaly greatly increases the electron-phonon coupling and induces a structural instability when the electronic density of states reaches a critical value. Our results reproduce the observed rise in T$_{c}$ from 11.2 K to 17.3 K as the nitrogen doping is increased. To further understand the important effect of the structural instability to the superconducting temperature, we model the Eliashberg spectral function with two contributions, one for the unstable phonons and the other for the unaffected phonons. Using the McMillan formula, we can predict the evolution of T$_{c}$ within this simple model that reproduces well our \textit{ab initio} results and the experimental data. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D33.00005: Competition between antiferromagnetism and superconductivity, electron-hole doping asymmetry and ``Fermi Surface" topology in cuprates Sandeep Pathak, Vijay Shenoy, Nandini Trivedi, Mohit Randeria We study the asymmetry between electron and hole doping in a 2D Mott insulator, and the resulting competition between antiferromagnetism (AF) and d-wave superconductivity (SC), using variational Monte Carlo for projected wave functions. We find that key features of the $T=0$ phase diagram, such as critical doping for SC-AF coexistence and the maximum value of the SC order parameter, are determined by a single parameter $\eta$ which characterizes the topology of the ``Fermi surface" at half filling defined by the bare tight-binding parameters. Our results give insight into why AF wins for electron doping, while SC is dominant on the hole doped side. We also suggest using band structure engineering to control the $\eta$ parameter for enhancing SC. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D33.00006: Density Matrix Renormalization Group Study of a Dynamic Hubbard Model, a Comparative Study Fatih Dogan, Frank Marisglio A one-dimensional model of holes locally coupled to pseudo spin degrees of freedom is studied using density matrix renormalization group. The model used in this talk is one in the family of dynamic Hubbard models. We look at density-density correlations, and frequency dependent functions to see existence and nature of the attraction of the holes in a electron-hole asymmetric system. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D33.00007: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 3:54PM - 4:06PM |
D33.00008: Valence bond glassy order and the pseudogap phase in underdoped high Tc cuprates Liang Ren, Ziqiang Wang Different origins of the pseudogap phenomena in underdoped high Tc cuprate have been proposed over the years, but a consistent theory has been challenging. We argue that the low-energy fluctuations of the valence bond, originating from the superexchange interaction, are pinned by the doping induced electronic disorder to give rise to a valence bond glass (VBG) pseudogap phase Using an extended t-J model within the Gutzwiller approximation, we show that the normal state VBG phase exhibits a genuine Fermi arc and a V-shaped average density of state at low energies. In the superconducting phase below Tc, the VBG can coexist and compete with an inhomogeneous d-wave superconductor, leading to the two-gap phenomena. We discuss the evolution of the local and momentum-space spectroscopy with doping and temperature, which capture the salient properties of the pseudogap phenomena and electronic disorder observed by recent ARPES and STM experiments. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D33.00009: On stability of odd-frequency superconducting state Dmitry Solenov, Ivar Martin, Dmitry Mozyrsky Odd-frequency pairing mechanism has been investigated for several decades. Nevertheless the properties of such superconducting phase as well as its thermodynamic stability have remained unclear. In particular it has been argued by numerous authors that the odd-frequency state is thermodynamically unstable, has an unphysical Meissner effect (at least within the mean-field approximation), and therefore can not exist as a homogeneous phase in equilibrium physical systems. We argue that such a conclusion is incorrect because it relies on an inappropriate assumption that the odd-frequency superconductor can be described by an effective Hamiltonian that breaks the U(1) symmetry. We show that the odd-frequency state can be appropriately formulated within the functional integral representation by using the effective action to describe such a superconducting state within the mean field approximation. We find that the odd-frequency superconductor is thermodynamically stable and exhibits ordinary Meissner effect, and therefore, in principle, it can be realized in equilibrium solid state systems. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D33.00010: Wave function for composite odd-frequency superconductors. Hari Dahal, Elihu Abrahams, Dmitry Mozyrsky, Yukio Tanaka, Alexander Balatsky Berezinskii proposed a new class of superconducting state that has an anomalous gap function that is odd function of frequency.$^{1}$ Following initial work of Berezinskii there has been growing interest in properties of such superconductors.$^{2-3 }$We propose a BCS-like wave function for an s-wave triplet odd-frequency superconductor. The wave function describes a condensate of spin-0 Cooper pair and spin-1 magnon; a composite order. By minimizing a Hamiltonian, suggested earlier in Ref. [3] to study the odd-frequency superconductor, we derive the quasiparticle dispersions, the self-consistent gap equation, and the density of states. We show that the quasiparticle excitations are gapless and the superconducting transition requires a critical coupling. References: \begin{enumerate} \item V. L. Berezinskii, JETP Lett. \textbf{20, }287 (1974). \item A. Balatsky, and E. Abrahams, PRB \textbf{45}, 13125 (1992). \item E. Abrahams, A. Balatsky, D. J. Scalapino and J. R. Schrieffer, PRB \textbf{52}, 1271(1995). \end{enumerate} [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D33.00011: Ginzburg Landau Theory for Cuprate Superconductivity Tiruppattur Ramakrishnan, Sumilan Banerjee, Chandan Dasgupta We propose and develop the consequences of a theory in which the free energy $F$ of a cuprate is expressed as a functional of the complex nearest neighbour spin singlet bond pair order parameter $\Delta_{ij} \exp{(i\phi_{ij})}$. $F$ is a sum of two terms $\sum_m(a \Delta_m^2+b\Delta_m^4)$, and $F_1= c\sum_{mn}\Delta_{mn}\cos{(\phi_m-\phi_n)}$; here $m$ is the site corresponding to $ij$ on the dual lattice (also square) and $m, n$ are nearest neighbours. The doping $x$ and temperature $T$ dependences of $a, b$ and $c$ are rationalized ( eg, $c\propto x$ for small $x$). The pseudogap ( due to incoherent bond pairs) and the parabolic $x$ dependence of $T_c$ ( AF ordering of the 2d-XY spin $\Delta_m\exp{(i\phi_m)}$ leading to d wave superconductivity) are described. The observed $C_v(T)$ behaviour is shown to be due to order parameter fluctuations. Detailed calculations of the vortex structure show a crossover from a Josephson like to a BCS like form with increasing doping, mirroring a similar change in superconductivity. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D33.00012: Levitation and lateral forces between a small magnet and superconducting sphere and the stability of the magnet H. Al-Khateeb, M. Alqadi, F. Alzoubi, B. Albiss, M. Hasan, N. Ayoub Using the dipole-dipole interaction model, we obtained analytical expressions for the levitation and lateral forces act on a small magnet for anti-symmetric magnet/spherical superconductor system. Breaking the symmetry of the system, allow as to study the lateral force which is important in the stability of the magnet above superconducting sphere in the Meissner state. Our formulas are written in terms of the radius of the superconductor as well as the height, the lateral displacement and the orientation of the magnetic moment of the magnet. We found that the levitation force is linearly dependent on the lateral displacement whereas the lateral force is independent of the lateral displacement. Moreover, the levitation and lateral forces are varying solinoudally with the polar and azimuthal angle of the orientation of the moment of the magnet. The stability of the magnet has been discussed for special orientations of the moment of the magnet. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D33.00013: HTSC Measurements Explained by Defended Superconductivity Theory John James Resonance between the superconducting gap and the low energy optical phonon mode creates a burst of coherent phonons which prevents the unstable superconducting state from collapsing. Surrounding lattice defects with charge states prevents interactions which would stop the coherent phonons from forming. This simple model of why Tc is high can explain many other phenomena, such as; insufficient density of states and instability in BCS model, variation of the gap energy in a single sample, the irreversibility line, sensitivity of Tc to impurities, high slope resistivity transition with impurities, isolation of magnetic impurities accept praeseodymium, long lived quasiparticles at low T, pseudogap, gapless pairs above Tc, proximity of phases, phase diagrams, low and high values of the tunneling gap, rise and fall of Tc with number of Cu layers, sensitivity to radiation damage at low temperature, normal state metal at the surface of samples cleaved at low temperature, eventual lowering of Tc with overdoping, remaining states in the photoemission gap, peak in the conductivity below Tc and many more. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D33.00014: A Dynamical Mean Field Study of the Three-Band Copper-Oxide Model Xin Wang, Luca de' Medici, Capone Massimo, Andrew J. Millis We apply the dynamical mean field theory to study the three-band Copper-Oxide Model related to the High-Tc Cuprates. Both continuous-time quantum Monte Carlo and Exact Diagonalization impurity solvers are used. The spectral function, mass enhancement and optical conductivity in both paramagnetic and antiferromagnetic case are computed. We determine the contribution of antiferromagnetic order to the gap observed in the undoped material. We show that in the paramagnetic Mott insulating regime the quasiparticle mass enhancement is larger for hole than for electron-doped compounds, but an opposite trend in matrix elements means that a the optical conductivity in these two cases is comparable. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D33.00015: Retarded vs instantaneous interactions in high-temperature superconductors. What is the glue? Bumsoo Kyung, David Senechal, A.-M.S. Tremblay In BCS theory, the phononic origin of the attraction that leads to Cooper pairs was confirmed by theoretical and experimental developments that clearly showed that the interaction was retarded and that the corresponding energy scales were associated with phonons. Using Cellular Dynamical Mean-Field Theory with exact diagonalization at $T=0$, we identify retardation effects in pairing and associate the corresponding energy scales with the spectral function of short-range spin fluctuations. These fluctuations are clearly seen in neutron and optical spectroscopy probes. Since the pairs have vanishing wave function at zero distance, the energy scale $U$ is absent from the pair dynamics. That dynamics can be monitored by the anomalous spectral weight. The Heisenberg exchange $J$ is a characteristic energy scale of that spectral weight and it appears in a manner analogous to what is found in mean-field theories. However, the anomalous spectral weight has additional structure caused by retardation. [Preview Abstract] |
Session D34: Superconductivity: Josephson Effects
Sponsoring Units: DCMPChair: Ulrich Welp, Argonne National Laboratory
Room: 404
Monday, March 16, 2009 2:30PM - 2:42PM |
D34.00001: Temperature and Field Dependence of the Emission of Terahertz Waves from Intrinsic Josephson Junctions Ulrich Welp, Alexei Koshelev, Lutfi Ozyuzer, Cihan Kurter, Masashi Tachiki, Kazuo Kadowaki, Taku Yamamoto, Ken Gray, Wai -K. Kwok We have recently succeeded in extracting coherent cw THz-radiation from intrinsic Josephson junctions in BSCCO [Science 318, 1291, (2007)]. An electromagnetic cavity resonance inside the sample generates a coherent state in which a large number of junctions are synchronized to oscillate in phase resulting in emission powers of up to 5 $\mu $W at frequencies up to 0.85 THz. The emission displays a non-monotonic temperature dependence with a sample dependent sharp maximum in the range of 25 to 45 K which we attribute to the interplay of self-heating effects and re-trapping of intrinsic junctions. Application of magnetic fields of less than 100 Oe parallel to the CuO$_{2}$ planes as well as perpendicular leads to the rapid suppression of the emission. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D34.00002: Spin-Josephson effect in antiferromagnetic tunnel junctions Dominique Chasse, A-M.S. Tremblay In the Josephson effect, coherent Cooper pair tunneling is driven by the phase difference between the superconducting order parameters on opposite sides of the junction. By analogy, differences in order parameters across a junction should lead to coherent tunneling of the condensed objects that exist in the broken symmetry state. To exhibit the generality of this phenomenon and make predictions from a realistic model, we study the case of a tunnel junction between two itinerant antiferromagnets. At the mean-field level, we find an equilibrium current of the staggered magnetization through the junction that is proportional the normal state conductance and to $\mathbf{S}_{L}\times \mathbf{S}_{R}$ where $\mathbf{S}_{L}$ and $\mathbf{S}_{R}$ are the staggered magnetizations on either sides. Microscopically, this effect comes from coherent tunneling of spin-one charge-zero particle-hole pairs that have a net wave vector equal to the antiferromagnetic one and zero spin projection along the direction of the order parameter. We explain similarities and differences with the standard DC and AC Josephson effects. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D34.00003: Angular dependence of the radiation power of a Josephson STAR-emitter Richard Klemm, Kazuo Kadowaki We calculate the angular dependence of the power of stimulated terahertz amplified radiation (STAR) emitted from a $dc$ voltage applied across a stack of intrinsic Josephson junctions. During coherent emission, we assume a spatially uniform $ac$ Josephson current density in the stack acts as a surface electric current density antenna source, and the cavity features of the stack are contained in a magnetic surface current density source. A superconducting substrate acts as a perfect magnetic conductor with $H_{||,ac}=0$ on its surface. The combined results agree very well with recent experimental observations. Existing Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ crystals atop perfect electric conductors could have Josephson STAR-emitter power in excess of 5 mW, acceptable for many device applications. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D34.00004: Surface effects in THz wave emission from intrinsic Josephson junctions Yoshihiko Nonomura Recently THz wave emission from intrinsic Josephson junctions without external fields [1] was observed experimentally. As possible states to characterize this emission, the McCumber-like state with little spatial dependence of electric fields (for the surface impedance $Z=1$) [2] and novel phase-kink state (for large and complex $Z$) [3] have been proposed. In the present study [4] it is numerically shown that both states are stationary and that the dynamical phase transition between these two states occurs as $Z$ is varied. The McCumber-like state is stable for low current and small $Z$. For higher current, the phase-kink state accompanied with symmetry breaking along the $c$ axis is stable even for $Z=1$, though strong emission in the vicinity of cavity resonance points only takes place for larger $Z$. Value of $Z$ is optimized for the strongest emission, and effect of surface roughness will also be discussed. [1] L.~Ozyuzer {\it et al.}, Science {\bf 318}, 1291 (2007); K. Kadowaki {\it et al.}, Physica C {\bf 468}, 634 (2008). [2] H.~Matsumoto {\it et al.}, Physica C {\bf 468}, 654, 1899 (2008). [3] S.~Lin and X.~Hu, Phys.\ Rev.\ Lett.\ {\bf 100}, 247006 (2008). [4] Y.~Nonomura, arXiv:0810.3756. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D34.00005: All-MgB$_{2}$ sandwich-type Josephson junctions with MgO barrier Ke Chen, Chenggang Zhuang, Qi Li, Ye Zhu, Paul Voyles, X. X. Xi Reproducible all-MgB$_{2}$ Josephson junctions have been made to meet the expectation for superconducting electronics that can work at above 20 K. The sandwich-type junctions were fabricated using MgB$_{2}$ electrodes grown by hybrid physical-chemical vapor deposition and MgO barrier deposited by RF magnetron sputtering. The $I-V$ characteristics show tunneling behavior with a small resistive shunt. The $I_{c}R_{n}$-product is 2.1 and 0.7 mV at 4.2 and 20 K, respectively, with temperature dependence following the theory qualitatively. The junctions exhibit good Fraunhofer pattern and Shapiro steps under applied magnetic field and microwave radiation, respectively. The $J_{c}$ of the junction varies exponentially with the barrier thickness, from 100 to 2$\times $10$^{5}$ A/cm$^{2}$. Transmission electron microscopy reveals both MgB$_{2}$ layers are epitaxially grown with $c$-axis parallel to the SiC (0001) substrate normal. With the same chip $J_{c}$ spead less than 10{\%}, this junction technology has the potential for MgB$_{2}$ circuits. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D34.00006: Direct observation of THz radiation from cylindrical structure of intrinsic Josephson junction system of Bi2212 M. Tsujimoto, T. Yamamoto, H. Minami, K. Kadowaki, M. Tachiki, U. Welp, W. Kwok Intense terahertz (THz) radiation was observed from a single crystalline high-$T_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi2212) system$^{1, 2)}$. We have performed various experiments on THz radiation with Bi2212 rectangular mesa structure fabricated by Argon-ion-milling and photolithography technique with changing the sample parameters. In this work, we report new experimental results obtained with samples which have a cylindrical structure fabricated by focused ion beam milling. The intense emission of electromagnetic radiation can be obtained in the return blanch only. The frequency is directly measured by FT-IR spectrometer to be $f$ = 0.474 THz in this particular sample of diameter with 90 $\mu $m. This frequency value is in good agreement with the fundamental cavity resonance mode frequency. 1) L. Ozyuzer et al., Science \textbf{318} (2007) 1291 2) K. Kadowaki et al., Physica C \textbf{468} (2008) 634-639 [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D34.00007: Sin(2$\phi )$ component in the current-phase relation of SFS Josephson junctions near the 0-$\pi $ transition M.J.A. Stoutimore, A.Yu. Rusanov, V.A. Oboznov, V.V. Bolginov, A.N. Rossolenko, V.V. Ryazanov, D.J. Van Harlingen We directly determined the Josephson current-phase relation (CPR) of superconductor-ferromagnet-superconductor (SFS) junctions by rf-SQUID interferometry, and corroborated it with measurements of the critical current as a function of temperature and magnetic field and rf-induced Shapiro steps in the current-voltage characteristics. Our Nb-Cu$_{47}$Ni$_{53}$-Nb trilayer junctions, with 2x2$\mu $m$^{2}$ area and 7nm CuNi thickness, show a transition with temperature from the usual Josephson 0-junction state to a $\pi $-junction state, defined by a phase difference of $\pi $ in the ground state, at temperatures between 1.5K and 3.5K. Near the transition, we observe second harmonics in the CPR, deviations from the usual Fraunhofer diffraction pattern and half-integer Shapiro steps, all consistent with a sin(2$\phi )$ component in the CPR. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D34.00008: Josephson interferometry evidence for order parameter with anisotropic and possibly complex phase in UPt3 Joel Strand, Dale Van Harlingen, J. B. Kycia, J. P. Davis, William Halperin The unconventional superconductor UPt3 exhibits many interesting properties. Among the most exciting is that it has two superconducting transitions, believed to arise from two different degenerate order parameters, at least one of which breaks time-reversal symmetry. The nature of the order parameter in each of these phases and the crossover between them is still not fully understood. We have fabricated Josephson junctions on various faces of high quality single crystals of UPt3. Measuring the magnetic field dependence of the critical current through these junctions produces patterns that reveal the intrinsic phase differences and underlying symmetry of the order parameter. Our results point to an order parameter with anisotropic and possibly complex phase that extends throughout the entire crystal. This is in contrast with our work on Sr2RuO4, which also breaks time-reversal symmetry but exhibits a distribution of dynamic chiral order parameter domains. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D34.00009: Interactions between Josephson Junction Metamaterials and Evanescent Waves Laura Adams, Steven Anlage Amplification of evanescent waves is an exciting, yet controversial application of negative index of refraction metamaterials in pursuit of creating a ``perfect lens''. We will describe evanescent wave amplification experiments using lossless metamaterials, i.e. arrays of Josephson junctions (JJ). The effects of input power, temperature, and dc magnetic field on JJ arrays below the cutoff frequency of a waveguide have been investigated. At low temperatures a pronounced, tunable microwave resonance emerges in transmission. This resonance has been systematically studied in terms of its transmission and reflection coefficients. In the regime between -45 and -25 dBm, we observe a non-hysteretic emission of microwave photons that reverberate at the same frequency. Amplification of these photons (parametric amplification) will also be described. This work was supported by the Intelligence Community Postdoctoral Fellowship program. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D34.00010: Direct test of pairing fluctuations in the pseudogap phase of an underdoped cuprate Jerome Lesueur, Nicolas Bergeal, Marco Aprili, Brigitte Leridon, Giancarlo Faini, Jean-Pierre Contour In underdoped cuprates, many experiments have provided evidence for the presence of a gap-like structure in the electronic excitations spectrum, in a region above the critical temperature and below a characteristic temperature T*. The origin of this so-called pseudogap is still hardly debated and the answer to this question turns out to be essential for the understanding of high-Tc superconductivity. One doesn't know if the pseudogap is related to superconductivity or to an order in competition. In the former case, it has been suggested that superconducting pairing fluctuations may be responsible for the partial suppression of electronic excitations. This remains to be tested experimentally, but most of the probes used to investigate the pseudogap are not sensitive to pairs and therefore cannot provide such a test. Here, we report for the first time on a direct test of pairing fluctuations in the pseudogap regime using a Josephson-like experiment. Our results shows that fluctuations survive only in a restricted range of temperature close to Tc (T-Tc$<$15K), and therefore cannot be responsible for the opening of the pseudogap at high temperature. Nature Physics 4, 608 - 611 (2008) [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D34.00011: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 4:42PM - 4:54PM |
D34.00012: Underdamped fluxon diffusion in a Josephson junction parallel array Kenneth Segall, Juan Mazo, Adam Dioguardi, Nikhil Fernandes We present experimental measurements and numerical simulations on the dynamics of fluxons in a parallel array of Josephson junctions. Fluxons trapped in a parallel array of Josephson junctions upon cooldown experience a periodic potential determined by the junction critical currents and the cell inductances. As shown in two recent papers [1,2], under certain conditions fluxons can move through the array in a series of noise-induced phase slips. This leads to a low-voltage diffusion branch in the current-voltage characteristics, similar to that in underdamped phase diffusion for a single junction. Unlike underdamped phase diffusion, however, this fluxon diffusion does not need frequency-dependent damping to occur. We demonstrate the existence of this state by direct measurements of the current-voltage characteristics and measurements of the switching current distribution of the array. 1. J.J. Mazo et al. Phys. Rev. B78, 174510 (2008) 2. K. Segall et al. http://arXiv.org/abs/0807.2978, to appear in J. Low Temp. Phys. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D34.00013: A tunable Josephson current in a Rashba Ring by Aharonov-Casher Phase Xin Liu, Mario F. Borunda , Xiong-Jun Liu, Jairo Sinova We study the interference effect induced by the Aharonov-Casher (A-C) phase on the transport between an asymmetrically confined two-dimensional electron ring system and the superconducting leads of a Josephson junction. The Josephson current and the Andreev levels are studied both analytically and numerically. Our results also predict oscillations in the Josephson current due to the A-C phase in the ring that can be tuned electrically via the spin-orbit (SO) interaction. Based on these oscillations, we propose a novel mechanism to observe the A-C phase in mesoscopic rings with gate tunable SO interaction attached to superconducting leads using the Josephson current oscillations. An applicable method to control the Josephson current by a tunable gate voltage can be realized due to these phenomena. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D34.00014: Enhanced thermal activation of a superconductor-normal metal quantum interference device Jian Wei, Paul Cadden-Zimansky, Venkat Chandrasekhar We measure the magnetoresistance and current-voltage characteristics (CVC) of a superconductor-normal metal quantum interference device in the form of a mesoscopic normal-metal loop in contact with two superconducting electrodes. Below the transition temperature of the superconducting leads, sharp switching from the zero-resistance state to a finite-resistance state is observed at half-integer flux quanta. The CVC in the finite-resistance state can be described by the Ambegaokar-Halperin (AH) theory of the effect of thermal fluctuations in Josephson junctions, but here this effect of thermal flucuations is greatly enhanced. The CVC in the zero-resistance state can be described by the Langer-Ambegaokar-McCumber-Halperin theory of thermally activated phase slips in one dimensional superconductors, but also with enhanced phase slipping rates, or equivalently, lowered energy barriers. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D34.00015: Static and dynamic effects in Superconducting/Normal metal/Superconducting long junctions Francesca Chiodi, Bertrand Reulet, H\'el\`ene Bouchiat We have studied four different Nb/Al long junctions at temperatures between 1.4 K and 4K (where the Al wire is still in the normal state). We have measured their low frequency current-voltage characteristics in the presence of an RF excitation (whose frequency varies from 100 kHz to 40 GHz). All the junctions show an important increase in critical current for frequencies above a threshold, which depends on the length of the normal wire. Is Thouless frequency playing a role? We have also studied the influence of geometry (narrow / square normal wire) on W/Au junctions. We have seen that the critical current/magnetic field curve changes from the well known Fraunhofer pattern (square wire) to a Gaussian dependence (narrow wire). We are now interested in dynamic properties of SN junctions: inductively coupling a multimode LC resonator to an AC SQUID we hope to define the behaviour of Andreev bound states when excitation time becomes lower than the typical diffusion time. [Preview Abstract] |
Session D35: Focus Session: Iron Pnictides and Other Novel Superconductors IV: General Experiment
Sponsoring Units: DMPChair: Rick Greene, University of Maryland
Room: 405
Monday, March 16, 2009 2:30PM - 3:06PM |
D35.00001: Iron Pnictide Superconductors: discovery and advances Invited Speaker: Superconducting transition in a layered ZrCuSiAs-type crystal was first reported for LaFePO in 2006 [1] and subsequently, a similar Tc was found for LaNiPO with the same crystal structure in 2007. However, Tc of these compounds reminded low ($\sim $4K). On February 23, 2008, our paper reporting a layered compound in LaFeAsO$_{1-x}$F$_{x}$(x=0.1) exhibiting a superconducting critical temperature Tc (mid-point) = 26K was published [3]. In this presentation I talk the background of this discovery and the subsequent advance in materials. The following points have been clarified to date; (1) Iron-based superconductors reported are 4-types crystal structures, the 1111[3], 122[4], 111[5], and 11 [6] type. All the high Tc iron-based superconductors contain a Fe square lattice and the Fe 3d orbitals dominate the Fermi-level. (2) The occurrence of a crystallographic transition accompanying anti-ferromagnetic to paramagnetic state in the parent compound is a requisite for a high Tc. (3) There exist a vast number of materials containing the Fe square lattice. (4). A partial substitution of Fe with other transition metal is possible without serious reduction of Tc. (4) A new insulating layer AEF (AE=Ca, Sr)was found to be effective in the 1111 phase [7]. (5) High pressure synthesis was effective to obtain the 1111 phases with higher Tc, (6) Epitaxial thin films exhibiting a Tc almost the same as that in the bulk were fabricated for CaFeAsO:Co[8]. Epitaxial thin films of LaFeAsO was recently reported as well [9]. \\[4pt] [1] Y.Kamihara et al. JACS,\textbf{ 28} (2006)10012, [2] T.Watanabe et al.Inorg.Chem,\textbf{46}(2007) 7719, [3 ]Y.Kamihara et al. J.Am.Chem.Soc.\textbf{130}(2008)3296., [4]M.Rotter et al. PRL, \textbf{101}(2008) 107006, [5] J.H.Tapp et al. PRB,\textbf{78}(2008)060505 [6] F.C.Hsu et al. PNAS,\textbf{105}(2008)14262., [7] S.Matsuishi et al. JACS \textbf{130}(2008)14428 [8] H.Hiramatsu et al. Appl.Phys.Express \textbf{1}(2008)101702, [9] H.Hiramatsu et al. APL.\textbf{ 93}(2008) 162504. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D35.00002: Materials and Physics in Pnictide Superconductors Invited Speaker: Superconductivity in the pnictides has shown itself to be very interesting and attractive. Some experimental results have revealed that the superconducting mechanism could be unconventional. In this talk I will survey our recent progress of both material synthesizing and physical properties of this rich family. We have made several major contributions to the synthesizing of new pnictide superconductors. (1) Fabrication of the hole doped RE$_{1-x}$Sr$_{x}$FeAsO samples (RE=La and Pr); (2) Fabrication of a series of new parent compounds DvFeAsF (Dv=divalent metals: Sr, Ca, Eu etc.) and many new superconductors with T$_{c}$ beyond 50 K by doping electrons into the system; (3) Invention of the new material (Sr$_{3}$Sc$_{2}$O$_{5})$Fe$_{2}$As$_{2}$ with rather large spacing distance between the FeAs planes. We have successfully grown the NdFeAsO$_{1-x}$F$_{x}$ and Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ single crystals. It is found that the anomalous electron scattering in the normal state cannot be simply attributed to the multiband effect. The influence given by the magnetic correlation may play an important role. Specific heat, lower critical field and point contact tunneling all indicate the unconventional superconductivity and multigap features, while the paring symmetry of the superconducting gap may be a non-trivial issue. In the 1111 phase, the superfluid density is rather low and contains probably a nodal feature. While in the 122 phase, both the superfluid density and the quasiparticle density of states is about 5-10 times higher than that in the 1111 phase. An s-wave component was found in the 122 phase. I will also report the measurements on anisotropy, critical current density, critical fields and vortex phase diagram. Small anisotropy, high upper critical field and fish-tail effect (in 122) were observed. All these suggest very good potential applications. In collaboration with Gang Mu, Zhaosheng Wang, Huiqian Luo, Huan Yang, Xiyu Zhu, Ying Jia, Yonglei Wang, Fei Han, Bing Zeng, Bing Shen, Cong Ren, Lei Shan. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D35.00003: Determination of the phase diagram of the electron doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Jiun-Haw Chu, James Analytis, Chris Kucharczyk, Ian Fisher Systematic measurements of the resistivity, heat capacity, susceptibility and Hall coefficient are presented for single crystal samples of the electron-doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. These data delineate an $x-T$ phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe$_2$As$_2$ at 134 K appears to split into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above $x \sim 0.025$, and appears to coexist with the broken symmetry state up to $x \sim 0.06$. The optimal T$_c$ appears to coincide with the Co concentration at which the magnetic/structural phase transitions are totally suppressed. Superconductivity is observed for a further range of Co concentrations, before being completely suppressed for $x \sim 0.18$ and above. The form of this $x-T$ phase diagram is suggestive of an association between superconductivity and a quantum critical point arising from suppression of the magnetic and/or structural phase transitions. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D35.00004: The magnetic and superconducting phase diagram of PrFeAsF$_{x}$O$_{1-x}$ Costel R. Rotundu, Stephen D. Wilson, Byron K. Freelon, Edith Bourret-Courchesne, Robert J. Birgeneau The electronic phase diagram of the newly discovered iron pnictide superconductors RFeAsO$_{1-x}$F$_{x}$ (R=rare-earth) is of great interest and with implications in the understanding of the nature of superconductivity (SC) itself. Predicted by \textit{ab initio} calculations [1] and pointed by resistivity measurements [2], the relevance of a quantum critical point remains controversial in the light of the structural phase transition between the magnetic SDW and SC [3]. We present a detailed magnetic and superconducting phase diagram of PrFeAsO$_{1-x}$F$_{x}$ as inferred from magnetic susceptibility and resistivity measurements. References: [1] G. Giovannetti \textit{et al.}, Physica B \textbf{403}, 3653 (2008) [2] R. H. Liu \textit{et al.}, Phys Rev Lett \textbf{101}, 087001 (2008) [3] H. Luetkens et al., cond.mat:0806.3533 [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D35.00005: Doping-Driven Collapse of the SDW Correlation Gap in SmFeAsO$_{1-x}$F$_{x}$ J.B. Kemper, Scott C. Riggs, Z. Stegen, G.S. Boebinger, R.D. Macdonald, F.F. Balakirev, Y. Kohama, A. Migliori, H. Chen, R.H. Liu, X.H. Chen We have investigated the Hall resistivity, $\rho_{xy}$ of polycrystalline SmFeAsO$_{1-x}$F$_{x}$ for four different fluorine concentrations from the onset of superconductivity through the collapse of the structural phase transition. For the two more highly-doped samples, $\rho_{xy}$ is linear in magnetic field up to 50 T with only weak temperature dependence, reminiscent of a simple Fermi liquid. For the lightly-doped samples with $x<0.15$, we find a low temperature regime characterized $\rho_{xy}(H)$ being both non-linear in magnetic field and strongly temperature dependent even though the Hall angle is small. The onset temperature for this non- linear regime is in the vicinity of the structural phase (SPT)/spin density wave (SDW) transitions. The temperature dependence of the Hall resistivity is consistent with a thermal activation of carriers across an energy gap. The evolution of the energy gap with doping is reported. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D35.00006: STM Investigation of the (001) surfaces of the Parent and Co-doped BaFe$_{2}$As$_{2}$ S. H. Pan, A. Li, D. R. Jayasundara, Y. Xuan, J. P. O'Neal, R. Jin, E. W. Plummer, R. Jin, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus We have used a UHV Low Temperature STM to study the surface structure of the parent and the Co-doped BaFe$_{2}$As$_{2}$ single crystals. Various STM images with different structural symmetry were observed. The dominant apparent surface structure is ($\surd $2 x$\surd $2)R45$^{O}$ for the parent compound and 1 x 2 stripe-like for the Co-doped ones. We will compare the different surface structures and discuss the identification of the atomic plane exposure and the possible origins for such variety in surface structure. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D35.00007: Electronic Structure on (001) Surface of BaFe$_{2}$As$_{2}$ Parent Compound Studied with Scanning Tunneling Spectroscopy D. R. Jayasundara, A. Li, Y. Xuan, J. P. O'Neal, S. H. Pan, R. Jin, E. W. Plummer, R. Jin, A. S. Sefat, M. A. McGuire, B. C. Sales, D. Mandrus Doping can drive some metallic pnictide compounds to superconducting phase. The microscopic mechanism of this phase transition has still not been understood. Starting with the parent compound, we have used a UHV Low Temperature STM to study the density of states on the (001) surface of single crystal BaFe$_{2}$As$_{2}$. The tunneling spectrum varies depending on the local environment. All the spectrums have the same background with density of states depression near the Fermi energy, but some of them show different anomalies. We attribute some of these anomalies to surface states. These results may provide useful information to those surface techniques other than STM. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D35.00008: Structural Investigation of the BaFe$_{2}$As$_{2}$(001) Surface Using LEED and STM Von Braun Nascimento, X.B. He, R. Jin, E.W. Plummer, T.Y. Chien, Biao Hu, Guorong Li, M.H. Pan, J.F. Wendelken, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus, Ang Li, Dilushan R. Jayasundara, Yi Xuan, Jared O'Neal, Shuheng Pan BaFe$_{2}$As$_{2}$, a parent compound to one of the newly discovered high-$T_{c}$ superconductors, presents very interesting physical properties such as a structural transition occurring prior to the formation of a spin density wave. It is surely of interest to investigate the effect of breaking the symmetry by creating a surface. Single crystals of BaFe$_{2}$As$_{2 }$were cleaved in vacuum exposing a (001) surface. Quantitative LEED I-V measurements and low-temperature STM topography revealed an ordered As surface with disorded Ba atoms present. LEED shows a clear (1 $\times $ 1) periodicity with a surface structure slightly different than the bulk. STM reveals a weak C(2 $\times $ 2) periodicity. We will explore the possibility that the C(2x2) STM image is electronic in origin. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D35.00009: Temperature and Spatial Dependence of the Superconducting and Pseudogap of NdFeAsO$_{0.86}$F$_{0.14}$. X.B. He, M.H. Pan, G.R. Li, J.F. Wendelken, R.Y. Jin, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus, E.W. Plummer Scanning tunneling microscopy/spectroscopy are used to investigate the superconducting gap and pseudogap of Fe based high-Tc superconducting material NdFeAsO0.86F0.14 at various temperatures from 17 K to 150 K. The superconducting gap (SG) in the tunneling spectra follows the BCS prediction and closes at Tc of the bulk material. Surprisingly, a pseudogap (PG) opens abruptly just above Tc and closes at 120 K, strongly suggesting that the SG and PG states have competing order parameters in contrast to the cuprates. The PG state may be related to spin fluctuations in the doped materials. Research was supported in part at ORNL by Laboratory Directed Research and Development funds and by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, US DOE. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D35.00010: Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron-Pnictide Superconductor BaFe$_{1.8}$Co$_{0.2}$As$_2$ Yi Yin, M. Zech, T.L. Williams, X.F. Wang, G. Wu, X.H. Chen, J.E. Hoffman We present an atomic resolution scanning tunneling spectroscopy study of superconducting BaFe$_{1.8}$Co$_{0.2}$As$_2$ single crystals in magnetic fields up to 9\, Tesla. At zero field, a single gap with coherence peaks at $\overline{\Delta}=6.25$\, meV is observed in the density of states. At 9\, T and 6\, T, we image a disordered vortex lattice, consistent with isotropic, single flux quantum vortices. Vortex locations are uncorrelated with strong scattering surface impurities, demonstrating bulk pinning. The vortex-induced sub-gap density of states fits an exponential decay from the vortex center, from which we extract a coherence length $\xi=27.6\pm 2.9$\, {\AA}, corresponding to an upper critical field $H_{c2}=43$\, T. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D35.00011: STM measurements on iron pnictides Zhanybek Alpichshev Some results of scanning tunneling microscopy/spectroscopy of cobalt doped pnictides at differrent doping levels are presented. [Preview Abstract] |
Session D36: Carbon Nanotubes: Low Temperature Electronic Properties
Sponsoring Units: DCMPChair: Xia Hong, Pennsylvania State University
Room: 408
Monday, March 16, 2009 2:30PM - 2:42PM |
D36.00001: Non-equilibrium tunneling spectroscopy in carbon nanotubes Nadya Mason, Yung-Fu Chen, Travis Dirks, Gassem Al-Zoubi, Norman Birge We report measurements of the non-equilibrium electron energy distributions in carbon nanotubes. Carbon nanotubes can be considered model one-dimensional systems whose transport is strongly affected by electron interactions. Using tunneling spectroscopy via a superconducting probe, we have studied electron energy distribution functions, and hence inelastic electron scattering, in nanotubes that have bias voltages applied between their ends. We find that at low temperatures, electrons interact weakly in nanotubes of a few microns channel length, independent of end-to-end conductance values. Surprisingly, the energy relaxation rate can increase substantially when the temperature is raised to only 1.5 K. In general, tunneling spectroscopy with a superconducting probe may be a powerful new tool for characterizing electron behavior in carbon nanotubes. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D36.00002: Method for determining the conductance tensor of quantum junctions from the ground state alone Armin Rahmani, Chang-Yu Hou, Claudio Chamon, Ian Affleck Conductance is related to dynamical correlation functions and is considered a non-equilibrium quantity. Here we propose a method to obtain the small-bias low-temperature conductance tensors of quantum junctions through equilibrium calculations such as time-independent DMRG. Using the dependence of a finite system ground state energy on the boundary conditions, we determine the junction conductance by finding the S-matrix. The method is applicable to interacting junctions connected to an arbitrary number of non-interacting leads. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D36.00003: Transition from the Sequential to the Resonant Tunneling in a Dissipative Environment Yuriy Bomze, Henok Mebrahtu, Ivan Borzenets, Alex Makarovski, Gleb Finkelstein We study the shape of the single-electron conductance peaks in a quantum dot coupled to a dissipative environment. In the regime of sequential tunneling through a single quantum level, the peak height increases as the temperature is lowered, although due to the dissipative environment it scales slower than the conventional $\sim $ 1/T. As the temperature is lowered further into the resonant tunneling regime, the peak width approaches saturation, while the peak height starts to decrease. To our knowledge, the non-monotonic peak height dependence on temperature is experimentally observed for the first time. We associate this behavior with coherent tunneling through a single quantum level in the presence of dissipative environment. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D36.00004: Possible electric-field induced one dimensional excitonic insulators in carbon nanotubes pairs Jay Sau, Marvin Cohen Recently there has been significant interest in the possibility of realizing excitonic insulator states in semiconducting systems in electric fields. Using a tight-binding formulation of the GW and Bethe-Salpeter methods parametrized from first-principles density functional theory calculations, we show that an electric field strength of 0.06 eV/\AA~ fails to close the quasiparticle gap of the system but closes the excitonic gap. This can cause a phase transition of the system into an excitonic phase where the ground state is populated with a quasi-one dimensional repulsive gas of excitons. Such a state provides a realization of a one-dimensional excitonic insulator phase with a spin degree of freedom which can lead to novel phases. We discuss some of the properties of the resulting excitonic phase and the transition and also discuss how similar properties may be observed in experiments on nanotube bundles. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D36.00005: A recursion formula for the local density of states in finite Luttinger liquids Sebastian Eggert, Imke Schneider The local density of states (LDOS) in quantum wires is one of the most central quantities for the experimental verification of the preditions from Luttinger Liquid theory. By now it has been well understood how boundaries lead to a crossover of powerlaws in the LDOS as a function of position and energy. It is also possible to calculate the LDOS for individual levels in finite wires analytically and numerically. However, the connection from finite wavefunctions to a semi-infinite powerlaw description remains unclear. We now present a simple recursion formula that ties together both limits and even allows to express the crossover of powerlaws in a closed analytic form in terms of hypergeometric functions. With the help of the formula it is now also possible to calculate the LDOS of long range interacting systems explicitly. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D36.00006: Superconducting Tunneling Spectroscopy of a Carbon Nanotube Quantum Dot Travis Dirks, Yung-Fu Chen , Nadya Mason, Norman Birge We report results on tunneling spectroscopy of a carbon nanotube quantum dot. Using a three-probe technique that includes a superconducting tunnel probe, we map out changes in conductance due to band structure, excited states, and applied bias. We also see features due to the unique nature of the superconducting probe, including enhancement of weak tunneling processes. In addition, we see~conduction inside the superconducting gap when an end to end bias is applied, which suggests some inelastic, possibly assisted, tunneling process inside the quantum dot. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D36.00007: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 3:54PM - 4:06PM |
D36.00008: A DMRG approach to impurities and interactions in carbon nanotubes Alexander Struck, Sebastian Reyes, Sebastian Eggert Carbon nanotubes (CNTs) are well suited to study strong electronic correlations in quasi-one-dimensional systems experimentally and theoretically. Of particular interest is the interplay of interactions between the conducting electrons and impurities in the nanotube. Impurities include the boundaries of short tubes as well as structural imperfections such as the Stone-Wales lattice distortion. Interactions can lead to different phases of the electron liquid, depending on their range and strength, and can produce quasi-localized ground states of e.g. the Mott insulator type or a charge density wave. Here we discuss a systematic approach using the density-matrix renormalization group (DMRG) method to treat a recently derived lattice model for a single-wall armchair CNT with short-range interactions and a Stone-Wales impurity. We show interaction driven modifications to the expected density patterns that can lead to anomalous Friedel oscillations around the impurity. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D36.00009: Zero-bias anomalies in multi-section carbon nanotube FETs Yanfei Yang, Georgy Fedorov, Serhii Shafraniuk, Rupert Lewis, Benjamin Cooper, Christopher Lobb, Paola Barbara Carbon nanotube field effect transistors (CNFETs) with high transparency contacts show maxima of differential conductance at zero bias voltage [1]. These zero-bias anomalies (ZBAs) occur at large negative gate voltages and in narrow gate voltage ranges (about 1 V wide). Our proposed explanation is superconductivity in the nanotubes, occurring when the gate voltage shifts the Fermi energy into van Hove singularities of the electronic density of states. Here we probe this scenario using 3 FETs fabricated from different sections of one semiconducting carbon nanotube. Source and drain electrodes were patterned by e-beam lithography to achieve FET lengths of 500 nm, 1500 nm and 7000 nm, respectively. All devices showed high transparency contacts to their Pd electrodes. We report the observation of pronounced ZBAs in the multi-section CNFETs, their magnetic field (up to 7 T) and temperature evolution, and the modulation on the ZBAs by Fabry-Perot oscillation. [1] J. Zhang et al., Zero-bias anomaly and possible superconductivity in single-walled carbon nanotubes, Phys. Rev. B, 74, 155414 (2006). [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D36.00010: Interacting resonant level side-coupled to a Luttinger liquid: Duality to resonant tunneling Moshe Goldstein, Richard Berkovits We study a model of a single level quantum dot side-coupled to a Luttinger liquid wire by both hopping and interactions. By canonical transformations and a Coulomb gas mapping, we prove a duality between this problem and that of resonant tunneling through a level connecting the edges of two wires with the inverse Luttinger liquid parameter $g$. The two systems thus have complementary transport properties: when one is conducting the other is insulating, and vice-versa. Using this result, as well as an exact solution at $g=2$ and Monte-Carlo simulations on the Coulomb gas, we fully characterize the system's conductance. It exhibits an anti-resonance as a function of the level energy, whose width vanishes (enhancing transport) as a power law at low temperatures and bias voltages for $g>1$, while diverging (suppressing transport) for $g<1$. Level population is shown to be either a linear, a power law, or a discontinuous function of a small level energy, depending on the parameters. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D36.00011: Wigner crystal \emph{vs.} Friedel oscillations in the 1D Hubbard model Stefan Soeffing, Michael Bortz, Sebastian Eggert We investigate the ground state density distribution of the Hubbard model in a finite one-dimensional wire. For weak interactions we find the expected Friedel oscillations, but for low filling a destinct Wigner crystal state can be observed. Although there cannot be a phase transition in a 1D system we observe a well-defined crossover into a Wigner crystal region with different physical behavior even for relatively weak short range interactions. The combination of Luttinger liquid theory and numerical Density Matrix Renormalization Group (DMRG) calculations allows a quantitative analysis of the crossover as a function of system length, lattice filling, and interaction strength. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D36.00012: Mott Insulating State in Ultra-clean Carbon Nanotubes Vikram Deshpande, Bhupesh Chandra, Robert Caldwell, Dmitry Novikov, James Hone, Marc Bockrath The Mott insulating state is a manifestation of strong electron interactions in nominally metallic systems. Using transport spectroscopy, we show that an energy gap exists in nominally metallic carbon nanotubes, and occurs in addition to the band-gap in small-band-gap nanotubes, indicating that carbon nanotubes are never metallic. This gap has a magnitude $\sim $10-100 meV and nanotube radius dependence $\sim $1/r, in good agreement with predictions for a nanotube Mott insulating state. We also observe neutral excitations within the gap, as predicted for this state. Our results underscore nanotubes' exceptional capabilities for studying correlated electron phenomena in 1D.\\ \\ Ref: V. V. Deshpande et al, \textit{Science} (in press) [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D36.00013: Superconductivity in thin films of boron-doped carbon nanotubes J. Haruyama, N. Murata, J. Reppert, A. Rao, T. Koretsune, S. Saito It is well known that the small mass of carbon can promote high transition temperature (T$_{c})$ in BCS-type superconductivity (SC). Recently, new carbon-based superconductors with order of T$_{c}$ of $\sim $10K [1, 2] were discovered and higher T$_{c}$ has been expected. In particular, the SC in a carbon nanotube (CNT) is attracting considerable attention [3]. We reported that entirely end-bonded multi-walled CNTs could show SC with T$_{c}$ = 12K, previously [4]. In contrast, it had problem in reproducibility, because correlation with carrier doping was not clarified. Moreover, none has succeeded substitutional carrier doping into CNTs and also revealed the correlation with SC. Here, we report on the Meissner effect found in thin films consisting of assembled boron-doped single-walled CNTs [5]. We reveal that only highly homogeneous CNT films consisting of low boron concentration leads to evident Meissner effect with T$_{c}$ = 12 K. The first-principles electronic-structure study of the $B$-SWNT strongly supports these results. [1] T. E. Weller et al., \textit{Nature Physics} 1, 39 (2005), [2] E. A. Ekimov et al., \textit{Nature} 428, 542 (2004), [3] M. Kociak et al., \textit{Phys. Rev. Lett.} 86, 2416 (2001), [4] I. Takesue, J.Haruyama, et al., \textit{Phys. Rev. Lett.} 96, 057001(2006), [5] N.Murata, J.Haruyama, et al., \textit{Phys.Rev.Lett.} 101, 027002 (2008) [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D36.00014: Fabrication and transport properties of size tunable single-walled carbon nanotube quantum dots Paul Stokes, Yodchay Jompol, Saiful I. Khondaker Single electron transistors (SETs) have attracted considerable attention because of their potential as a building block for quantum based nanoelectronic devices. However fabrication of reproducible and controllable quantum dot sizes that can operate at high temperature is challenging. We developed a novel technique for the fabrication of size tunable and controllable quantum dot using single-walled carbon nanotube (SWNT) [1]. Our technique is based on the formation of two tunnel barriers of controllable separation by naturally bending SWNT at the edges of a raised local gate. A SWNT is placed on a local Al/Al$_{2}$O$_{3}$ bottom gate of width $L$, and then contacted with Pd source and drain electrodes of 1 $\mu $m separation on Si/SiO$_{2}$ substrates. The Al gate serves three purposes: (i) it acts as a ``mechanical template'' to define two tunnel barriers at the edges by naturally bending the nanotube due to van der Walls interactions with the substrate, (ii) the width of the gate defines the size ($L)$ of the quantum dot, and (iii) it acts as a local bottom gate to control the operation of the SET device. Using this approach we fabricated SETs of different sizes down to 50 nm. We present detailed fabrication procedures and low temperature transport studies of these SET devices. [1] P. Stokes and S. I. Khondaker, APL \textbf{92}, 262107 (2008). [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D36.00015: Conductance of a Conjugated Molecule with Carbon Nanotube Contacts Nicolas Bruque, Khalid Ashraf, Thomas Helander, Roger Lake Quantitative predictions of the energy levels is a well-known weakness of density functional theory (DFT). To understand the HOMO level alignment of a $\pi $-cruciform molecule [1] with the Fermi level of a carbon nanotube (CNT) contact, we have performed quantum chemical calculations of the adiabatic ionization potential (IP) of the central molecule. The adiabatic IP of the molecule is -5.86 eV. The image charge potential, calculated using our fully self-consistent DFT - Recursive Green Function (RGF) approach, is 0.7 eV. Treating the image potential as a self-energy correction to the IP, the HOMO energy level is at -5.16 eV which is comparable to the intrinsic CNT Fermi level at -5 eV. The above considerations of the energy level alignments, combined with the DFT-RGF analysis of the molecular orbitals and transmission spectrum, indicate that the HOMO resonance lies within the 50 meV energy window created by the experimental source-drain bias. This appears to be the most likely scenario that would give rise to the relatively small resistance of 6 M$\Omega $. \newline 1. X. Guo, et. al. Science, \textbf{311}, 356 (2006). [Preview Abstract] |
Session D37: Focus Session: Fundamental Developments in Density Functional Theory III
Sponsoring Units: DCPChair: Alberto Castro, Free University of Berlin
Room: 409
Monday, March 16, 2009 2:30PM - 3:06PM |
D37.00001: Bounds on the correlation energy of Coulomb interacting systems: How negative does $E_c$ get, and what does this imply for approximate density functionals? Invited Speaker: The indirect part of the Coulomb interaction energy of a three-dimensional many-fermion system has a lower bound in terms of a power of the particle density, known as the Lieb-Oxford bound. This bound can be reformulated as a bound on the correlation energy, and in this reformulated version is an ingredient in the construction of many modern density functionals. In this talk, I describe several recent investigations and refinements of this bound: (i) an empirical analysis strongly suggesting that the bound can be tightened without loosing its universality [collaboration: Mariana Odashima], (ii) the construction of a particle-number dependent version of the bound and an exploration of its consequences for PBE GGA [collaboration: Mariana Odashima and Sam Trickey], (iii) a simplified scaling derivation of the power law in the bound, and its application to construct similar bounds also for one- and two-dimensional systems [collaborators: C\'{e}sar Proetto, Esa R\"{a}s\"{a}nen and Stefano Pittalis], and (iv) a connection between the Lieb-Oxford bound and common hybrid functionals, providing an alternative rationale for why these functionals work, as well as a possible route for the construction of improved beyond-GGA functionals [collaborator: Mariana Odashima]. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D37.00002: Van der Waals Interactions in Density-Functional Theory. Invited Speaker: The application of conventional GGA, and meta-GGA, density functionals to van der Waals complexes is fraught with difficulties. Conventional functionals do not contain the physics of the dispersion interaction. To make matters worse, the exchange part alone can yield anything from severe over-binding to severe over-repulsion depending on the choice of functional. We rectify these problems by - adding a dispersion term with nonempirical C6, C8, and C10 dispersion coefficients (the Becke-Johnson dispersion model), and - selecting a GGA exchange functional (PW86, also nonempirical) that gives excellent agreement with exact Hartree-Fock repulsion curves. The result is a simple GGA+dispersion theory giving excellent noble-gas pair interaction energies for He through Kr with only two adjustable parameters in the dispersion cutoff. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D37.00003: Van der Waals interactions in density functional theory Invited Speaker: The van der Waals density functional which we introduced half a decade ago\footnote{M. Dion et al.\ Phys.\ Rev.\ Lett. \textbf{92}, 246401 (2004).} and its self-consistent generalization\footnote{T. Thonhauser et al., Phys.\ Rev.\ B \textbf{76}, 125112 (2007).} will be briefly reviewed. There are many collaborators in the application review that will follow, not only those who worked in the physics department at Rutgers% \footnote{Maxime Dion, Aaron Puzder, T. Thonhauser, Valentino R. Cooper, Shen Li, Eamonn Murray, Lingzhu Kong, and Kyuho Lee.} and at Chalmers,% \footnote{Henrik Rydberg, Svetla Chakarova-K{\"a}ck, Jesper Kleis, Elsebeth Schr{\"o}der, Per Hyldgaard, and Bengt I. Lundqvist.} but also at Denmarks Technical University,% \footnote{Andrei Kelkkanen, Poul G. Moses, Jesper Kleis, and Bengt I. Lundqvist.} the chemistry department at Rutgers,% \footnote{Konhoa Li, Jing Li, Yves Chabal, and Wilma K. Olson.} and most recently at the University of Texas at Dallas.% \footnote{Nour Nijem and Yves Chabal.} I will expand on our recent review article,\footnote{D. C. Langreth et al., J. Phys.\ Cond.\ Mat.\ (in press).} which hopefully will be published before the present talk, and include applications by other groups not listed below. If possible, I will also review results from a more recent collaboration to study nucleosomal DNA and beyond. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D37.00004: Accurate van-der-Waals interactions from (semi)-local density functional theory Alexandre Tkatchenko, Matthias Scheffler Non-covalent forces, such as hydrogen bonding and van der Waals (vdW) interactions, are crucial for the formation, stability and function of molecules and materials. At present, vdW interactions can only be satisfactorily accounted for by high-level quantum-chemical wave function or by the Quantum Monte Carlo (QMC) method. In contrast, (semi)-local DFT and Hartree-Fock approximation fail for the description of vdW forces. We present a parameter-free method for describing the long-range vdW interaction in (semi)-local DFT. The leading $C_6$ coefficients are derived from the electron density of a molecule/solid and accurate reference values for the free atoms. The mean absolute error in the $C_6$ coefficients is 5.5\% when compared to accurate experimental values for 1225 intermolecular pairs. We show that the $C_6$ coefficients depend strongly on the bonding type and geometry of molecules/solids. Finally, we analyze the vdW radii and the damping function in the $C_6R^{-6}$ correction method for DFT calculations. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D37.00005: Efficient van der Waals density functional interactions Jose M. Soler, Guillermo Roman-Perez The LDA and GGA functionals are the non empirical methods of choice for large system calculations, but they cannot describe nonlocal dispersion forces. This limits severly their application to many systems of large interest, like molecular solids and liquids, physisorbed molecules, and interactions between biological molecules. Several schemes have been proposed to add ad-hoc atom-atom or atom-electron potentials. But dispersion is an electron-electron correlation effect, that must be described by an appropriate electron density functional, such as that proposed by Dion et al (PRL 92, 246401 (2004)). It is a true universal and general-purpose DFT functional that describes semiquantitatively the week dispersion interactions, without compromising the accuracy of the best GGA functionals for stronger bonds. Its direct evaluation for large molecular systems is very expensive, however, because it requires a double integral in real space. We present a new implementation that avoids this $N^2$ scaling by applying Fourier convolution techniques to an accurately interpolated kernel. The resulting method scales as $N\log N$ and it allows to perform vdW-DFT simulations of essentially any system that can be simulated with GGA. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D37.00006: The discontinuous nature of the exchange-correlation functional -- critical for strongly correlated systems Paula Mori-Sanchez, Aron Cohen, Weitao Yang Standard approximations for the exchange-correlation functional have been found to give big errors for the linearity condition of fractional charges, leading to delocalization error, and the constancy condition of fractional spins, leading to static correlation error. These two conditions are now unified for states with both fractional charge and fractional spin: the exact energy functional is a plane, linear along the fractional charge coordinate and constant along the fractional spin coordinate with a line of discontinuity at the integer. This sheds light on the nature of the derivative discontinuity and calls for explicitly discontinuous functionals of the density or orbitals that go beyond currently used smooth approximations. This is key to understand the physics of strongly correlated systems within DFT, for example the band-gap of Mott insulators. [arXiv:0809.5108] [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D37.00007: Advances in Local Hybrid Functionals Alexey Arbuznikov, Martin Kaupp, Hilke Bahmann Local hybrids\footnote{ Jaramillo, J; Scuseria, G. E.; Ernzerhof, M. \textit{J. Chem. Phys.} \textbf{2003}, $118$, 1068} provide a promising new generation of exchange-correlation functionals for the simultaneous accurate description of various properties (atomization energies, reaction barrier heights,\footnote{ Kaupp, M.; Bahmann, H.; Arbuznikov, A. V. \textit{J. Chem. Phys.}, \textbf{2007}, $127$, 194102} NMR chemical shifts,\footnote{ Arbuznikov, A. V.; Kaupp, M. \textit{Chem. Phys. Lett.} \textbf{2007}, $442$, 496} energetics of transition-metal systems, etc.) Compared to traditional (global) hybrids (e.g., B3LYP), instead of a constant exact-exchange admixture, local hybrids employ a position-dependent one. The latter is governed by a so-called \textit{local mixing function} (LMF), and this is the crucial quantity controlling the performance of local hybrids. Here we present and compare new results obtained with LMFs derived both in a semiempirical way and using \textit{ab initio} considerations, e.g., the adiabatic connection formalism.\footnote{ Arbuznikov, A. V.; Kaupp, M. \textit{J. Chem. Phys.} \textbf{2008}, $128$, 214107} The former approach yields better results, while the latter brings valuable insights into the performance and limits of local hybrids. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D37.00008: Calculation of functional derivatives with respect to the external potential Nick Sablon, Tim Fievez, Frank De Proft, Paul W. Ayers, Paul Geerlings Apart from its many computational advantages, density functional theory (DFT) presents a conceptual framework for the reactivity and stability interpretation of chemical systems. The central idea is to identify chemical concepts with first and higher order (functional) derivatives of the electronic energy with respect to the number of electrons N and the external potential $v\left( r \right)$. The local interpretation of chemical reactivtiy is generally done with the Fukui function and the dual descriptor. These reactivity indices are usually calculated by a finite difference approach, which is enitrely justified in an exact theory. Practical DFT calculations make however use of approximate exchange-correlation functionals for which the DFT concepts can only correctly be obained by an effective evaluation of the electronic energy derivatives. A recent methodology for the calculation of functional derivatives with respect to $v\left( r \right)$ is presented.\footnote{P.W. Ayers, F. De Proft, A. Borgoo and P. Geerlings, J. Chem. Phys. 126, 224107 (2007).}$^,$\footnote{T. Fievez, N. Sablon, F. De Proft, P.W. Ayers and P. Geerlings, J. Chem. Theory Comput. 4, 1065 (2008).} Results are shown for a wide range of molecules among which substituted benzenes. A reactivity description of the alkaline earth oxides' (100) surface is expounded on as well. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D37.00009: Simple Illustration of Partition Theory Adam Wasserman, Morrel Cohen, Kieron Burke, Roberto Car In Partition Theory (PT) [M.H. Cohen and A. Wasserman, J.Phys. Chem. A 2007, 111, 2229], the density of a system is decomposed exactly into a superposition of the densities of its parts through the introduction of a common \textit{partition potential} acting on each of the parts as if they were isolated. In this talk we illustrate PT on a simple one-dimensional model of a heteronuclear diatomic molecule. We show that a sharp definition for the charge of the fragments emerges from PT, and that the ensuing population analysis can be used to study how charge redistributes during dissociation. By studying the preservation of the shapes of the parts as different parameters of the model are varied, we address the issue of transferability of the parts. We find good transferability within the chemically meaningful parameter regime, raising hopes that PT will prove useful in chemical applications. [Preview Abstract] |
Session D38: Focus Session: The Chemical Physics of Biological and Biologically-inspired Solar Energy Harvesting III
Sponsoring Units: DCPChair: Martin Plenio, Imperial College
Room: 410
Monday, March 16, 2009 2:30PM - 3:06PM |
D38.00001: Exciton migration and fluorescence quenching in photosystem II Invited Speaker: When exposed to excess light illumination photosynthetic organisms switch into a photoprotective quenched state where the excess energy is safely dissipated as heat. It was recently discovered that the main light-harvesting complex of plants, LHCII, plays a key role in the dissipation of excess energy. Here we demonstrate that the excitation kinetics in the quenched state can be described by a simple model, which assumes specific trapping centers to be present in the system [1]. In order to explain the experimental results exciton-exciton annihilation is taken into account. To verify the effectiveness of the non-photochemical quenching center, possessing a short lifetime, in preventing the excess excitations from reaching the reaction center, the studies of the excitation quenching depending on positioning and origin of the quencher in the antenna complexes are also considered. \\[4pt] [1] N. E. Holt, D. Zigmantas, L. Valkunas, X.-P. Li, K. K. Niyogi, G. R. Fleming, \textit{Science} 307, 433 (2005). [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D38.00002: Environment-assisted quantum transport in photosynthetic complexes. Invited Speaker: Transport phenomena at the nanoscale are of interest due to the presence of both quantum and classical behavior. In this work, we demonstrate that quantum transport efficiency can be enhanced by a dynamical interplay of the system Hamiltonian with the pure dephasing dynamics induced by a fluctuating environment. This is in contrast to fully coherent hopping that leads to localization in disordered systems, and to highly incoherent transfer that is eventually suppressed by the quantum Zeno effect. We study these phenomena in the Fenna-Matthews-Olson protein complex as a prototype for larger photosynthetic energy transfer systems. We also show that disordered binary tree structures exhibit enhanced transport in the presence of dephasing. We address the question of the role of coherence in the energy transfer in the FMO complex and discuss details about the theoretical modeling of photosynthetic oomplexes and organic photovoltaic materials. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D38.00003: Engineering Efficient Exciton Energy Transfer in Artificial Arrays Leslie Vogt, Alejandro Perdomo, Semion Saikin, Alan Aspuru-Guzik A critical component of light harvesting devices is efficient transfer of excitonic energy. Biological systems have optimized this process over time for the particular molecular components involved. Understanding this energy transfer in model arrays will allow us to engineer new materials for solar cell technology. In particular, we explore a perturbative approach to optimize both coherent and incoherent transport in small arrays. By following the evolving coherences and populations over time using a density matrix formalism, we gain an intuition about the importance of coherent processes in exciton transfer in natural and designed light harvesting systems. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D38.00004: Coherent Excitonic Transfer in the Fenna Matthews Olson Complex Gregory Engel Evidence for a purely quantum mechanical mechanism of energy transfer in photosynthetic complexes was discovered in the Fenna-Matthews-Olson complex of \textit{Chlorobium tepidum in 2007}. The quantum beating phenomenon observed in this complex is now much better understood. Specifically, detailed, testable microscopic models for the mechanism of this energy transfer have emerged, and precise quantum dynamical models now predict that this mechanism accounts for approximately one quarter of the energy transferred at room temperature. Further, new data indicate that this mechanism is not specific to FMO, but manifests in reaction centers of purple bacteria and antenna complexes of higher plants. A new experimental effort to observe quantum coherence at room temperature will be discussed. Specifically, by comparing population transfer rates and coherence transfer quantum beating signals, we caluclate the fraction of the energy moving through the wave-like mechanism. Further, by studying the temperature dependence of the energy transfer, we elucidate the microscopic mechanism for wavelike energy transfer and be able to comment on the robustness of the mechanism. Are light harvesting proteins delicately ``tuned'' by evolution to support coherence transfer or should any proteinaceous environment support this mechanism? Details of the experimental apparatus, results and future experiments will be presented. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D38.00005: Excitation transport in open quantum systems: the role of environmental correlations. Mohan Sarovar, Yuan-Chung Cheng, Birgitta Whaley The recent discovery of quantum coherent phenomena in photosynthetic complexes [Engel et.al., Nature, 446, 782 (2007), Lee et. al., Science, 316, 1462 (2007)] has prompted several studies into the efficiency of transport processes in open quantum systems. Several of these studies have revealed a subtle interplay between coherent and decoherent dynamics in the overall efficiency of transport in these open systems. Some have shown that decoherence can improve efficiency. However all studies have used simple uncorrelated models of decoherence that are not accurate for photosynthetic complex environments, which are known to be spatially and temporally correlated. In this work we investigate the role of environmental correlations in quantum transport in open systems and show that the exact nature of the correlations can have a large impact on the efficiency of energy harvesting. We illustrate our results using the Fenna-Matthews-Olsen photosynthetic complex. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D38.00006: Local Correlation Calculations Using Standard and Renormalized Coupled-Cluster Methods Piotr Piecuch, Wei Li, Jeffrey Gour Local correlation variants of the coupled-cluster (CC) theory with singles and doubles (CCSD) and CC methods with singles, doubles, and non-iterative triples, including CCSD(T) and the completely renormalized CR-CC(2,3) approach, are developed. The main idea of the resulting CIM-CCSD, CIM-CCSD(T), and CIM-CR-CC(2,3) methods is the realization of the fact that the total correlation energy of a large system can be obtained as a sum of contributions from the occupied orthonormal localized molecular orbitals and their respective occupied and unoccupied orbital domains. The CIM-CCSD, CIM-CCSD(T), and CIM-CR-CC(2,3) algorithms are characterized by the linear scaling of the total CPU time with the system size and embarrassing parallelism. By comparing the results of the canonical and CIM-CC calculations for normal alkanes and water clusters, it is demonstrated that the CIM-CCSD, CIM-CCSD(T), and CIM-CR-CC(2,3) approaches recover the corresponding canonical CC correlation energies to within 0.1 {\%} or so, while offering savings in the computer effort by orders of magnitude. By examining the dissociation of dodecane into C$_{11}$H$_{23}$ and CH$_{3}$ and several lowest-energy structures of the (H$_{2}$O)$_{n}$ clusters, it is shown that the CIM-CC methods accurately reproduce the relative energetics of the corresponding canonical CC calculations. [Preview Abstract] |
Session D39: Focus Session: Noise and Fluctuations in Biochemical Networks
Sponsoring Units: DBP GSNPChair: Jayajit Das, Ohio State University
Room: 411
Monday, March 16, 2009 2:30PM - 3:06PM |
D39.00001: Stochasticity in cell biology: Modeling across levels Invited Speaker: Effective modeling of biological processes requires focusing on a particular level of description, and this requires summarizing de details of lower levels into effective variables and properly accounting for the constrains that other levels impose. In the context of stochasticity in gene expression, I will show how the details of the stochastic process can be characterized by a few effective parameters, which facilitates modeling but complicates interpretation of current experiments. I will show how the resulting noise can provide advantageous or deleterious phenotypic fluctuation and how noise control in the copy number control system of plasmids can change the selective pressures. This system illustrates the direct connection between molecular dynamics and evolutionary dynamics. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D39.00002: Mean-field vs. Stochastic Models for Transcriptional Regulation Ralf Blossey, Claudiu Giuraniuc We introduce a minimal model description for the dynamics of transcriptional regulatory networks. It is studied within a mean-field approximation, i.e., by deterministic ode's representing the reaction kinetics, and by stochastic simulations employing the Gillespie algorithm. We elucidate the different results both approaches can deliver, depending on the network under study, and in particular depending on the level of detail retained in the respective description. Two examples are addressed in detail: the repressilator, a transcriptional clock based on a three-gene network realized experimentally in E. coli, and a bistable two-gene circuit under external driving, a transcriptional network motif recently proposed to play a role in cellular development. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D39.00003: The stochastic spectral analysis of transcriptional regulatory cascades Andrew Mugler, Aleksandra M. Walczak, Chris H. Wiggins Modeling the dynamics of biological networks while respecting the intrinsic stochasticity requires accounting for intrinsic fluctuations arising from the low copy count of the constituent particles. Traditional simulation-based approaches to computing the probability distribution, rather than by direct solution of its master equation, are fundamentally limited by long runtimes and the need to estimate a the distribution from samples. We obviate both limitations by directly solving for the distribution using a fast and accurate method that exploits the natural basis of the uncoupled problem from the same class. We illustrate our method on a ubiquitous biological example: linear signaling cascades. The huge efficiency gains permit optimization of information transmission over input and regulatory parameters, revealing design properties of the most informative cascades. We find, for threshold regulation, that a cascade of strong regulations converts a unimodal input to a bimodal output, that multimodal inputs are no more informative than bimodal inputs, and that a chain of “DC” up-regulations outperforms a chain of “AC” down-regulations. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D39.00004: Tuning stochastic transition rates in a bistable genetic network. Vijay Chickarmane, Carsten Peterson We investigate the stochastic dynamics of a simple genetic network, a toggle switch, in which the system makes transitions between the two alternative states. Our interest is in exploring whether such stochastic transitions, which occur due to the intrinsic noise such as transcriptional and degradation events, can be slowed down/speeded up, without changing the mean expression levels of the two genes, which comprise the toggle network. Such tuning is achieved by linking a signaling network to the toggle switch. The signaling network comprises of a protein, which can exist either in an active (phosphorylated) or inactive (dephosphorylated) form, and where its state is determined by one of the genetic network components. The active form of the protein in turn feeds back on the dynamics of the genetic network. We find that the rate of stochastic transitions from one state to the other, is determined essentially by the speed of phosphorylation, and hence the rate can be modulated by varying the phosphatase levels. We hypothesize that such a network architecture can be implemented as a general mechanism for controlling transition rates and discuss applications in population studies of two differentiated cell lineages, ex: the myeloid/erythroid lineage in hematopoiesis. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D39.00005: Optimizing information flow in small genetic networks Aleksandra M. Walczak, Gasper Tkacik, Curtis G. Callan, William Bialek Many of the biological networks inside cells can be thought of as transmitting information from the inputs (e.g., the concentrations of transcription factors or other signaling molecules) to their outputs (e.g., the expression levels of various genes). On the molecular level, the relatively small concentrations of the relevant molecules and the intrinsic randomness of chemical reactions provide sources of noise that set physical limits on this information transmission. Given these limits, not all networks perform equally well, and maximizing information transmission provides a candidate design principle from which we might hope to derive the properties of real regulatory networks. As a starting point, I will consider the simple case of one input transcription factor that controls many genes. I will discuss the properties of these specific small networks that can transmit the maximum information. Concretely, I will show how the form of molecular noise drives predictions not just of the qualitative network topology but also the quantitative paramaters for the input/output relations at the nodes of the network. In an attempt to link these general theoretical considerations to real biological systems, I will illustrate the predictions on the example of transmission of positional information in the early development of the fly embryo. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D39.00006: Characterizing noise in genetic oscillatory systems Byungjoon Min, Kwang-Il Goh, In-mook Kim Quantitative understanding of fluctuations in genetic circuits is crucial for understanding living systems. Despite the recent advances in the subject, however, fluctuations in non-stationary activities such as molecular oscillations have not been much investigated yet. Here we quantify the fluctuations in periods and amplitudes of oscillation and the noise propagation in the genetic oscillatory system, the repressilator, using exact stochastic simulation. At the single protein level, we found that the fluctuation in oscillation amplitudes is larger than that in oscillation periods. Noise propagation is studied in terms of the correlations in the successive periods and amplitudes, respectively, which decay exponentially down the regulatory cascades. We then study the extended repressilator system to investigate the effect of extra component and identify the combinatoric regulation pattern that reduces the fluctuations in oscillatory activities significantly. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D39.00007: Purely stochastic binary decisions in cell signaling models without underlying deterministic bistabilities Maxim N. Artyomov, Jayajit Das, Mehran Kardar, Arup Chakraborty Detection of different extra-cellular stimuli leading to functionally distinct outcomes is common in cell biology, and is often mediated by differential regulation of positive and negative feedback loops that are a part of the signaling network. For cellular responses stimulated by small numbers of molecules, the stochastic effects are important. Therefore, we studied the influence of stochastic fluctuations on a simple signaling model with dueling positive and negative feedback loops. The class of models we have studied is characterized by single deterministic steady states for all parameter values, but the stochastic response is bimodal; a behavior that is distinctly different from models studied in the context of gene regulation. For small numbers of signaling molecules, stochastic effects result in a bimodal distribution for this quantity, with neither mode corresponding to the deterministic solution; i.e., cells are in ``on'' or ``off'' states, not in some intermediate state. For a large number of molecules, the stochastic solution converges to the mean-field result. When fluctuations are important, we find that signal output scales with control parameters ``anomalously'' compared to mean-field predictions. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D39.00008: Fitness effects of fluctuations in biochemical networks Invited Speaker: The concentration of many cellular components fluctuates not only as a response to external and internal inputs but also due to random birth and death events of individual molecules. This biochemical noise affects the capacity of every individual cell in a population to respond and adapt to the environment. While the sources and effects of biochemical fluctuations on individual cells have been intensively studied, the effects of noise on the growth rate of a population of cells are much less understood. We present a model of the cell cycle in which the growth and division of individual cells are coupled with the noisy dynamics of their internal components. The model allows us to compute the contribution of the biochemical noise to the average growth rate of a population of cells as a function of the noise strength and the correlation time of the fluctuations. We show that, due to fluctuations, the growth rate of a population of cells is always larger than the average growth rate of a individual cell and can be larger even than a corresponding deterministic model. In most relevant cases it is assumed that the average concentration of a cellular component is close to a value that maximizes the population growth as given by the external, environmental, conditions and the internal cellular regulation. In such cases we show that contribution of fluctuations to the growth rate is negative and increases with the sensitivity of the biochemical networks to the noise sources and the noise correlation time. We also discuss how the selection pressure due to fluctuations affects the structure and parameters of genetic regulatory networks. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D39.00009: Effects of delay and noise in a negative feedback regulatory motif Matteo Palassini, Marta Dies The small copy number of the molecules involved in gene regulation can induce nontrivial stochastic phenomena such as noise-induced oscillations. An often neglected aspect of regulation dynamics are the delays involved in transcription and translation. Delays introduce analytical and computational complications because the dynamics is non-Markovian. We study the interplay of noise and delays in a negative feedback model of the p53 core regulatory network. Recent experiments have found pronounced oscillations in the concentrations of proteins p53 and Mdm2 in individual cells subjected to DNA damage. Similar oscillations occur in the Hes-1 and NK-kB systems, and in circadian rhythms. Several mechanisms have been proposed to explain this oscillatory behaviour, such as deterministic limit cycles, with and without delay, or noise-induced excursions in excitable models. We consider a generic delayed Master Equation incorporating the activation of Mdm2 by p53 and the Mdm2-promoted degradation of p53. In the deterministic limit and for large delays, the model shows a Hopf bifurcation. Via exact stochastic simulations, we find strong noise-induced oscillations well outside the limit-cycle region. We propose that this may be a generic mechanism for oscillations in gene regulatory systems. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D39.00010: Individuals in the crowd: studying bacterial quorum-sensing at the single-cell level Pablo Delfino Perez, Jonathan Young, Elaine L. Johnson, Stephen J. Hagen Like many bacterial species, the marine bacterium \textit{Vibrio fischeri} can detect its own population density through a quorum sensing (QS) mechanism. The bacterium releases a small molecule signal -- the autoinducer (AI) -- into its environment: high AI concentration indicates high population density and triggers a genetic switch that, in \textit{V.fischeri}, leads to bioluminescence. Although the QS behavior of bulk cultures of \textit{V.fischeri }has been extensively studied, little is known about either the response of individual cells to AI signal levels or the role of noise and local diffusion in QS signaling. We have used a photon-counting camera to record the luminescence of individual \textit{V.fischeri} cells immobilized in a flow cell and subject to varying concentrations of AI. We observe that light output by individual cells varies not only with bulk AI concentration, but also over time, between cells, with local (micron-scale) population density, and even with the flow rate of the medium. Most of these variations would not be evident in a bulk culture. We will present an analysis of this heterogeneity at the cell level and its implications for the role of noise in QS signaling. [Preview Abstract] |
Session D40: Nucleic Acids: Packaging, Ejection and Translocation
Sponsoring Units: DBPChair: Xiangyun Qiu, National Institutes of Health
Room: 412
Monday, March 16, 2009 2:30PM - 2:42PM |
D40.00001: Anomalous scaling of nano-pore translocation times of structured biomolecules Malcolm McCauley, Robert Forties, Ulrich Gerland, Ralf Bundschuh Translocation through a nano-pore is a new experimental technique to probe physical properties of biomolecules. A bulk of theoretical and computational work exists on how the main observable, the time to translocate a single molecule, depends on the length of the molecule for unstructured molecules. Here, we study the same problem but for RNA molecules for which the breaking of the secondary structure is the main barrier for translocation. To this end, we calculate the mean translocation time of single-stranded RNA through a nanopore of zero thickness and at zero voltage for many randomly chosen RNA sequences. We find the translocation time to depend on the length of the RNA molecule with a power law. The exponent changes as a function of temperature and exceeds the naively expected exponent of two for purely diffusive transport at all temperatures. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D40.00002: DNA Physical Mapping via the Controlled Translocation of Single Molecules through a 5-10nm Silicon Nitride Nanopore Derek Stein, Walter Reisner, Zhijun Jiang, Nick Hagerty, Charles Wood, Jason Chan The ability to map the binding position of sequence-specific markers, including transcription-factors, protein-nucleic acids (PNAs) or deactivated restriction enzymes, along a single DNA molecule in a nanofluidic device would be of key importance for the life-sciences. Such markers could give an indication of the active genes at particular stage in a cell's transcriptional cycle, pinpoint the location of mutations or even provide a DNA barcode that could aid in genomics applications. We have developed a setup consisting of a 5-10 nm nanopore in a 20nm thick silicon nitride film coupled to an optical tweezer setup. The translocation of DNA across the nanopore can be detected via blockades in the electrical current through the pore. By anchoring one end of the translocating DNA to an optically trapped microsphere, we hope to stretch out the molecule in the nanopore and control the translocation speed, enabling us to slowly scan across the genome and detect changes in the baseline current due to the presence of bound markers. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D40.00003: The Effects of Bio-functionalization on Solid-state Nanopore Transport -- Theory and Experiments on DNA Yaling Liu, Abhijit Ramachandran, Samir M. Iqbal Solid-state nanopore channels have been reported recently to show selectivity for various target bio-molecules. The surfaces of nanopore channels are functionalized to achieve such selectively. The organic molecule coatings alter the behavior of molecular transport as well as change surface energies, chemical and physical properties, and make these more bio-compatible. We present theoretical considerations of DNA-modified nanopore channels which treat the functional molecules on the surface as a combination of series of potential sites. The potential function depends on the physical interactions of two ssDNA molecules. The simulated DNA trajectories and translocation speeds under various test conditions are consistent with the reported experimental data. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D40.00004: Nanopores as a Single-Molecule Probe for Protein-DNA Complexes A.R. Hall, S.W. Kowalczyk, R.M.M. Smeets, N.H. Dekker, C. Dekker In recent years, solid state nanopores have emerged as a productive novel technique for molecular biophysics. The electrophoretic motion of single molecules through these small-scale structures can offer insights into both conformation and charge structure. Here, we apply the method to the RecA nucleoprotein filament - a conformation where proteins polymerize along the entire length of a double-stranded DNA. This offers a unique geometry and charge structure which we probe through a combination of translocation experiments and optical tweezer measurements. We discuss conductance blockade events that are notably larger (12 nS) than those measured for bare dsDNA (1 nS), and present force spectroscopy data showing a high level of charge screening in solution ($>$90{\%}). [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D40.00005: Computational model of controlled translocation of DNA molecule through a nanopore membrane with tunable electrostatic potential Alexey Nikolaev, Maria Gracheva We present results of computational modeling of controllable DNA translocation through a nanopore in a thin electrically tunable membrane composed of two layers of n-type and p-type semiconductor materials. Membrane potential biases are used to obtain distinct electrostatic potential landscapes. The membrane-DNA system is immersed in a biased electrolyte solution under bias to induce DNA translocation. A simple charges-and-springs model is used to model polynucleotide molecule. We compare electrostatic potential landscapes of the membrane with one and more potential extrema and show how electrostatic potential landscape in the nanopore alters the control over the molecule translocation. In particular, we specify different conditions under which DNA nucleotides can be translocated through the nanopore one by one in both directions as well as paused in the nanopore. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D40.00006: Heterogeneity in Retroviral Nucleocapsid Protein Function Christy Landes Time-resolved single-molecule fluorescence spectroscopy was used to study the human T-cell lymphotropic virus type 1 (HTLV-1) nucleocapsid protein (NC) chaperone activity as compared to that of the HIV-1 NC protein. HTLV-1 NC contains two zinc fingers with each having a CCHC binding motif similar to HIV-1 NC. HIV-1 NC is required for recognition and packaging of the viral RNA and is also a nucleic acid chaperone protein that facilitates nucleic acid restructuring during reverse transcription. Because of similarities in structures between the two retroviruses, we have used single-molecule fluorescence energy transfer to investigate the chaperoning activity of HTLV-1 NC protein. The results indicate that HTLV-1 NC protein induces structural changes by opening the transactivation response (TAR)-DNA hairpin to an even greater extent than HIV-1 NC. However, unlike HIV-1 NC, HTLV-1 NC does not chaperone the strand-transfer reaction involving TAR-DNA. These results suggest that despite its effective destabilization capability, HTLV-1 NC is not as effective at overall chaperone function as is its HIV-1 counterpart. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D40.00007: Energetics of genome ejection from phage revealed by isothermal titration calorimetry Meerim Jeembaeva, Bengt Jonsson, Martin Castelnovo, Alex Evilevitch It has been experimentally shown that ejection of double-stranded DNA from phage is driven by internal pressure reaching tens of atmospheres. This internal pressure is partially responsible for delivery of DNA into the host cell. While several theoretical models and simulations nicely describe the experimental data of internal forces either resisting active packaging or equivalently favoring spontaneous ejection, there are no \textit{direct} energy measurements available that would help to verify how quantitative these theories are. We performed \textit{direct} measurements of the enthalpy responsible for DNA ejection from phage $\lambda $, using \textit{Isothermal Titration Calorimetry}. The phage capsids were ``opened'' \textit{in vitro} by titrating $\lambda $ into a solution with LamB receptor and the enthalpy of DNA ejection process was measured. In his way, enthalpy stored in $\lambda $ was determined as a function of packaged DNA length comparing wild-type phage $\lambda $ (48.5 kb) with a shorter $\lambda $-DNA length mutant (37.7 kb). The temperature dependence of the ejection enthalpy was also investigated. The values obtained were in good agreement with existing models and provide a better understanding of ds- DNA packaging and release mechanisms in motor-packaged viruses (e.g., tailed bacteriophages, Herpes Simplex, and adenoviruses). [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D40.00008: Inhibition of DNA ejection from bacteriophage by Mg$^{+2}$ counterions Seil Lee, Cathy V. Tran, Toan T. Nguyen The problem of inhibiting viral DNA ejection from bacteriophages by multivalent counterions, especially Mg$^{+2}$ counterions, is studied. Experimentally, it is known that MgSO$_4$ salt has a strong and non-monotonic effect on the amount of DNA ejected. There exists an optimal concentration at which the least DNA is ejected from the virus. At lower or higher concentrations, more DNA is ejected from the capsid. We propose that this phenomenon is the result of DNA overcharging by Mg$^{+2}$ multivalent counterions. As Mg$^{+2}$ concentration increases from zero, DNA net charge changes from negative to positive. The optimal inhibition corresponds to the Mg$^{+2}$ concentration where DNA is neutral. At lower/higher concentrations, DNA genome is charged. It prefers to be in solution to lower its electrostatic self-energy, which consequently leads to an increase in DNA ejection. Our theory fits experimental data well. The strength of DNA-DNA short range attraction, mediated by Mg$^{+2}$, is found to be $-$0.003 $k_B$T per nucleotide base. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D40.00009: Osmotic pressure: resisting or promoting DNA ejection from phage? Internal capsid-pressure dependence of viral infection Alex Evilevitch, Meerim Jeembaeva, Sarah Koester, Martin Castelnovo, David Weitz Recent\textit{ in vitro }experiments have shown that DNA ejection from phage can be partially stopped by surrounding osmotic pressure when ejected DNA is digested by DNase I on the course of ejection. We argue in this work by combination of experimental techniques (UV absorbance, pulse-field electrophoresis, and cryo-EM) that intact genome ($i.e$. \textit{undigested}) ejection in a crowded environment is, on the contrary, enhanced or eventually complete with the help of a pulling force resulting from DNA condensation induced by the osmotic stress itself. This demonstrates that in vivo, the osmotically stressed cell cytoplasm will promote phage DNA ejection rather than resisting it. While, in vitro, the ejection depends sensitively on internal pressure within the virus capsid, the effect of internal pressure on infection of bacteria is unknown. We use microfluidics to monitor individual cells and determine the distribution of lysis due to infection as the capsid pressure is varied. The lysis probability decreases markedly with decreased capsid pressure. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D40.00010: Charting the Structure and Energetics of Packaged DNA in Bacteriophages Xiangyun Qiu, Donald C. Rau, V. Adrian Parsegian, Li Tai Fang, Charles M. Knobler, William M. Gelbart Many bacterial viruses resort to pressure in order to infect bacteria, e.g., lambda phage stores its dsDNA genome at surprisingly high pressure and then uses this pressure to drive delivery of the genome. We report on a biophysical interrogation of the DNA configuration and pressure in lambda phage by combining structural and thermodynamic measurements with theoretical modeling. Changes in DNA organization in the capsid are monitored using solution small angle x-ray scattering (SAXS). We vary the DNA-DNA repulsion and DNA bending contributions to the capsid pressure by changing salt concentrations and packaged length, and augment SAXS data with osmotic stress measurements to elicit the evolving structure and energetics of the packaged DNA. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D40.00011: Stabilising the Herpes Simplex Virus capsid by DNA packaging Gijs Wuite, Kerstin Radtke, Beate Sodeik, Wouter Roos Three different types of Herpes Simplex Virus type 1 (HSV-1) nuclear capsids can be distinguished, A, B and C capsids. These capsids types are, respectively, empty, contain scaffold proteins, or hold DNA. We investigate the physical properties of these three capsids by combining biochemical and nanoindentation techniques. Atomic Force Microscopy (AFM) experiments show that A and C capsids are mechanically indistinguishable whereas B capsids already break at much lower forces. By extracting the pentamers with 2.0 M GuHCl or 6.0 M Urea we demonstrate an increased flexibility of all three capsid types. Remarkably, the breaking force of the B capsids without pentamers does not change, while the modified A and C capsids show a large drop in their breaking force to approximately the value of the B capsids. This result indicates that upon DNA packaging a structural change at or near the pentamers occurs which mechanically reinforces the capsids structure. The reported binding of proteins UL17/UL25 to the pentamers of the A and C capsids seems the most likely candidate for such capsids strengthening. Finally, the data supports the view that initiation of DNA packaging triggers the maturation of HSV-1 capsids. [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D40.00012: Effect of ions on polymer ejection dynamics from viral capsids Issam Ali, Davide Marenduzzo, Julia Yeomans We present simulations investigating the impact of adding ions on the dynamics of semifexible (DNA-like) polymers ejecting from spherical viral capsids. We find that when the DNA charge is less screened, due to, for example, the addition of monovalent ions like Na+, the resulting electric interactions give rise to larger ejection forces, speeding up the ejection process. The results suggest that DNA ejection can be controlled by tuning the salt concentration in the environment, in agreement with recent experiments. We also observe that the DNA structure inside the capsid changes when electrical forces are present, tending to become more spool-like. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D40.00013: Investigating Nanopore Spatial Resolution Using Locally Coated RecA-dsDNA Filaments A.R. Hall, S.W. Kowalczyk, C. Dekker The translocation of molecules through nanometer-scale apertures has garnered much attention as a future sequencing method. Many challenges remain, including the high spatial and temporal resolution needed to do so. We examine the spatial limits of these measurements by translocating partially complexed~RecA nucleoprotein filaments. These are dsDNA polymerized with discrete RecA protein patches of random length, ranging from a few monomers to full coverage (average length $\sim $10 kbp). With these molecules, we use nanopores for the first time to map the location of features along the length of a single molecule. We show that resolution of less than 500 bp is achieved and discuss the implications on translocation measurements. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D40.00014: Electrically Gated Solid State Nanopores Zhijun Jiang, Walter Reisner, Derek Stein We are exploring the use of electrically functionalized solid-state nanopores for controlling the transport of ions and single DNA molecules in solution. We have integrated annular gate electrodes inside solid-state nanopores that can electrostatically adjust both the polarity and the density of the inner surface charge. An applied gate potential can thereby influence the density of mobile counter-ions inside a pore at low salt concentrations. Our theoretical calculations show that a 0.1 V change in the gate potential can change the pore conductance by more than a factor of 5, making the nanopore behavior similar to that of a transistor. Furthermore, the electrostatic interaction between the nanopore surface and negatively charged DNA molecules can be probed in the regime of Double-layer overlap. A negatively charged inner nanopore surface should repel DNA, and limit its possibility to insert into the nanopore. Positive surface charges, on the other hand, will attract DNA, and translocation should be favored. We seek to electrostatically control the translocation of DNA through the nanopore, and thereby mimic single-molecule regulatory capabilities of biological nanopores. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D40.00015: Structural transitions in packing of a semi-flexible chain confined in a sphere Artem Levandovsky, Leonid Pryadko, Roya Zandi We study phases and phase transitions (crossovers) between phases of a semi-flexible polymer chain confined in a spherical cavity. Such a problem is relevant to DNA or RNA packaging in viruses whose organization is characterized by both simplicity and economy. The confinement involves both energetic and entropic effects controlled by the stiffness of the chain, its length and diameter, and the sphere radius. Formation of different packing configurations and structural changes in these configurations is studied with a non-local ``cluster'' Monte Carlo method. We introduce several order parameters characterizing different packing symmetries and compute the corresponding probability distributions. This allows us to reconstruct the Landau free energies for these order parameters, and thus develop a simple theory of packing transitions. [Preview Abstract] |
Session D41: Hidden Order and Quantum Criticality in Heavy Fermions
Sponsoring Units: DMP DCMPChair: Johnpierre Paglione, University of Maryland
Room: 413
Monday, March 16, 2009 2:30PM - 2:42PM |
D41.00001: Incommensurate spin resonance in URu2Si2. Jian Xin Zhu, Alexander Balatsky, Athanasios Chantis, Hari Dahal, David Parker The nature of the hidden order (HO) in URu2Si2 below T$_{HO}$ = 17.5K has been a puzzle for a long time. Here we propose to search for the spin resonance as a tool to elucidate the nature of the HO. We consider inelastic neutron scattering in URu2Si2 and argue that a gap in the fermion spectrum will produce an incommensurate spin resonance at Q* = (1$\backslash $pm 0.4, 0,0) at $\omega _{res}$ = 4-6 meV. We assume that the HO gap is due to a particle-hole condensate that connects nested parts of the Fermi surface with nesting vector {\$}Q\^{}* . The predicted behavior of the spin susceptibility is strikingly similar to the phenomenology of resonance peaks in high-T{\_}c and heavy fermion superconductors. The energy of the resonance peak scales with T$_{HO}$ $\omega _{res} \quad \sim $ 4 k{\_}BT$_{HO}$ [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D41.00002: Thermodynamical Properties across Quantum Critical Points Jianda Wu, Lijun Zhu, Qimiao Si Quantum critical points (QCPs) are of extensive current interest, in part because they strongly influence the physical properties at finite temperatures. Thermodynamic properties have recently been used as a means to probe the energy scales in the quantum critical heavy fermion metals [1]. In addition, the divergence of the Gruneisen ratio (thermal expansion to specific heat) or its magnetic analog at any QCP, theoretically predicted a few years ago [2], has been observed in a growing list of quantum critical materials. In this work, we study the entropy as a function of control parameter in several theoretical models for quantum criticality. We explicitly demonstrate that the entropy is maximized near the QCP, which is compatible with the divergence of the Gruneisen ratio exactly at the QCP. When the control parameter is a magnetic field, we also study the field dependence of the isothermal magnetization and other magneto-thermal properties. [1] P. Gegenwart et al, Science 315, 969 (2007); [2] L. Zhu et al, PRL 91, 066404 (2003). [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D41.00003: Cyclotron Resonance in the Hidden-Order Phase of Ultraclean URu$_2$Si$_2$ Single Crystals T. Shibauchi, K. Hashimoto, K. Ikada, S. Tonegawa, H. Shishido, Y. Haga, T. D. Matsuda, Y. Onuki, H. Yamagami, Y. Matsuda In the heavy fermion compound URu$_2$Si$_2$, the hidden-order transition occurs at 17.5 K, whose origin is still an enigma. Of primary importance is elucidating the electronic structure of the hidden-order phase. Here we report the first observation of cyclotron resonance in the ultraclean crystals of URu$_2$Si$_2$ with residual resistivity ratio $RRR=670$. The magnetic-field dependence of the microwave surface impedance in the Azbel'-Kerner geometry shows clear cyclotron resonance lines whose line width has characteristic temperature dependence consistent with the transport measurements. In addition to the bands which have been previously identified in the quantum oscillation measurements, we newly find the missing band with the heaviest mass, which can account for the large specific heat coefficient in the hidden-order phase of URu$_2$Si$_2$. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D41.00004: Reconstruction of the Fermi Surface Deep inside the Hidden-Order Phase of Very Clean URu$_2$Si$_2$ Y. Matsuda, H. Shishido, T. Shibauchi, K. Hashimoto, Y. Haga, T.D. Matsuda, Y. Onuki, T. Sasaki, T. Oizumi, N. Kobayashi, T. Takamasu, K. Takehana, Y. Imanaka The nature of the hidden order (HO) phase in URu$_2$Si$_2$ is a long standing mystery in heavy-fermion physics. It has been shown that the HO phase is destroyed at $H_0$=36~T ($T=0$) and several new phases appear above $H_0$. Here we studied the low temperature/high field phase of very clean URu$_2$Si$_2$ single crystals ($RRR$=670) by the transport properties. We find that the Hall resistivity jumps at $H_h$=22~T well inside the HO phase and new quantum oscillations appear at high fields starting slightly below $H_h$. These results indicate a reconstruction of the Fermi surface and a possible phase transition well inside the HO phase. The present results provide strong evidence that the HO transition is described by an itinerant rather than a localized electron picture. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D41.00005: Probing the Hidden Order in URu$_2$Si$_2$ by Impurity Doping Seung-Ho Baek, Nicholas Curro, M. Graf, A. Balatsky, Eric Bauer, Jason Cooley, Jim Smith URu$_2$Si$_2$ exhibits a clear broken symmetry ground state at 17.5 K, but the nature of the order parameter has not been known for more than two decades. Motivated by the fact that Rh doping in this compound induces antiferromagnetism, indicating that the hidden order is closely related with the antiferromagnetism, we studied $^{29}$Si NMR with varying Rh concentration. This $^{29}$Si NMR study reveals that the antiferromagnetism arises from the local suppression of the hidden order by Rh doping. We propose that the antiferromagnetism emerges as a result of the local suppression of the hidden order yet only within the long range hidden order phase. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D41.00006: Electronic structure model of the hidden order and Fermi surface gapping in URu$_{2}$Si$_{2}$ Peter Oppeneer, Saad Elgazzar, Jan Rusz, Michi-To Suzuki, John Mydosh The hidden order (HO) in the heavy-fermion superconductor URu$_{2}$Si$_{2}$ has been studied for more than 20 years, without that the nature of this unusual phase could be uncovered. We present a microscopic explanation for the mechanism of the hidden order, on the basis of state-of-the-art electronic structure calculations. In particular, we show that our calculations explain very well all the known properties of the paramagnetic and large moment antiferromagnetic (LMAF) phases. Exploiting the known experimental equivalence between the Fermi surface properties of the LMAF and HO phases, we identify the Fermi surface ``hot spots'' where a Fermi surface instability is lifted through spontaneous symmetry breaking, causing a surprisingly large Fermi surface gapping. We quantify that symmetry breaking through collective modes of antiferromagnetic moment excitations can induce a substantial Fermi surface gapping that consistently explains the transport properties and entropy loss of the HO phase. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D41.00007: Heavy Fermion and non-Fermi Liquid Properties vs Size: From the Micro to the Nano G.R. Stewart, J.S. Kim, K. Samwer Y. Y. Chen et al. have studied nanoparticles of several systems, including CePt$_{2}$ [1]. We report here the specific heat, C, down to 0.05 K and $\chi$ to 2 K as a function of size for several Ce- and U-heavy Fermion and non-Fermi liquid (nFl) systems, including UBe$_{13}$ and Rh-doped CeRu$_{2}$Si$_{2}$. Using dry sieves (for larger particles) and aqueous suspension/filtration techniques using Isopore$^{TM}$ filters (for smaller particles), size gradations from 45-53 $\mu$ (essentially bulk) down to 0.6-1.2 $\mu$ were studied. One goal was to study the evolution of nFl behavior vs decreasing size at a Quantum Critical Point, where the spatial extent of the fluctuations should become infinite, or at least larger than the particle at some size. Ce-systems showed the beginning of Kondo peak behavior in C below 3 $\mu$, however it was still possible to determine the evolution of the intrinsic low temperature nFl C/T $\sim$ logT in Rh-doped CeRu$_{2}$Si$_{2}$ as a function of decreasing size to address this goal. The effect of size on superconductivity and m$^{*}$ in UBe$_{13}$ will also be discussed.\\[0pt] [1] Y. Y. Chen et al., Phys. Rev. Lett. 98, 157206 (2007). [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D41.00008: Quantum criticality in the Bose-Fermi Kondo model and the quantum-to-classical mapping Stefan Kirchner, Qimiao Si The Bose-Fermi Kondo model (BFKM) occurs as the effective quantum impurity model within the Extended Dynamical Mean Field Theory (EDMFT) for quantum critical heavy fermion metals. The quantum critical point (QCP) of the BFK is therefore related to the one in the Kondo lattice model, the relevant low-energy model for heavy fermion compounds. There have been indications that the QCP of the BFKM cannot be described in terms of a local O(3)-symmetric $\phi^4$-theory as predicted by the quantum-to-classical mapping, but the issue remains to be settled. In this work we demonstrate that the quantum-to-classical mapping for the spin-isotropic SU(N) BFKM breaks down in a large N limit[1]. We also show that this feature is associated with the Berry phase term of the spin path integral and therefore persists for finite N [2]. We analyzse the influence of this breakdown on the dynamic scaling properties of the Kondo lattice obtained through the EDMFT, and also discuss the connection of our results with those of the Ising-anisotropic BFKM. [1] L. Zhu, S. Kirchner, Q. Si and A. Georges, PRL 93, 267201 (2004). [2] S. Kirchner and Q. Si, arXiv:0808.2647 (2008). [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D41.00009: Thermal expansion and magnetostriction of the heavy fermion antiferromagnet YbAgGe G.M. Schmiedeshoff, A.W. Lounsbury, S.J. Tracy, S.L. Bud'ko, P.C. Canfield YbAgGe is a stoichiometric heavy fermion compound that exhibits antiferromagnetic order and field induced quantum criticality.~ We will discuss this behavior, and present a unified phase diagram of this compound in the T-H plane, in light of our recent thermal expansion and magnetostriction measurements.~ We find a remarkable agreement between thermodynamic, transport and microscopic measurements on this model system. Work at Occidental College was supported by the National Science Foundation under DMR-0704406. Work at the Ames Laboratory was supported by the Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D41.00010: Magnetic-field-dependence of the YbRh$_2$Si$_2$ Fermi surface Patrick Rourke, Alix McCollam, Gerard Lapertot, Georg Knebel, Jacques Flouquet, Stephen Julian Magnetic-field-induced changes of the Fermi surface play a central role in theories of the exotic quantum criticality of YbRh$_2$Si$_2$. We have carried out de Haas--van Alphen measurements in the magnetic field range 8 T~$\leq H \leq$~16~T, and directly observe field-dependence of the extremal Fermi surface areas. Our data support the theory that a low-field ``large'' Fermi surface, including the Yb 4$f$ quasi-hole, is increasingly spin-split until a majority-spin branch undergoes a Lifshitz transition and disappears at $H_0 \approx 10$~T, without requiring 4$f$-localization at $H_0$. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D41.00011: Electron Spin Resonance in a Kondo lattice, Pedro Schlottmann Until recently it was commonly believed that due to the broad linewidth electron spin resonance (ESR) could not be observed in heavy-fermion compounds. This was proven to be wrong, since an ESR signal was found$^1$ in single crystals of YbRh$_2$Si$_2$ as well as in other systems. Recently, Abrahams and W\"olfle$^2$ studied the ESR signal of a heavy fermion band within the framework of the Anderson lattice. They obtained that the heavy mass in conjunction with ferromagnetic fluctuations can lead to narrow resonances, and concluded that the observed ESR in YbRh$_2$Si$_2$ is due to the heavy fermion conduction states and not the Yb localized moments. Here I study the ESR linewidth for localized moments within the framework of the Kondo lattice model. An ESR signal can only be observed if the Kondo temperature is sufficiently small. In addition, to obtain an observable signal short-range ferromagnetic correlations between the localized spins are necessary, which may lead to a bottleneck situation, that narrows the linewidth. It is concluded that from ESR data alone it is not possible to distinguish if the resonance is due to localized spins or conducting heavy electron spins. \par\noindent Work supported by the Department of Energy under grant No. DE-FG02-98ER45797. \par\noindent $^1$ J. Sichelschmidt {\it et al.}, Phys. Rev. Lett. {\bf 91}, 156401 (2003). \par\noindent $^2$ E. Abrahams and P. W\"olfle, Phys. Rev. B {\bf 78}, 104423 (2008). [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D41.00012: Cyrstal Synthesis of Novel Yb-Pt-Pb Phases Carlos Marques, Yuri Janssen, Marcus Bennet, Moo Sung Kim, Keeseong Park, Peter Khalifah, Meigan Aronson We have used flux techniques to explore the Yb-Pt-Pb ternary phase diagram, and have grown a number of intermetallic compounds including YbPt, Yb$_{3}$Pt$_{5}$, and the new~Yb$_{5}$Pt$_{9}$, YbPt$_{2}$, Yb$_{3}$Pt$_{4}$ and Yb$_{2}$Pt$_{2}$Pb, as well as Yb$_{3}$Pt$_{5}$Si and YbPtSi. The crystal structure of these different compounds will be compared. A particular focus has been the synthesis of single crystals of quantum critical antiferromagnet (AF) Yb$_{3}$Pt$_{4}$, and we show that it is possible to~synthesize crystals which are large enough for neutron diffraction measurements. Laue patterns and neutron rocking curves along with other methods show that these cyrstals are of very high quality. Initial results of neutron diffraction and inelastic scattering experiments on single Yb$_{3}$Pt$_{4}$ crystals and arrays of multiple Yb$_{3}$Pt$_{4}$ crystals will be presented. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D41.00013: Neutron scattering studies on Yb$_{3}$Pt$_{4}$ Y. Janssen, M.C. Bennett, C. Marques, L. Wu, M.S. Kim, K.S. Park, Q. Huang, J.Y. Li, Y. Chen, J.W. Lynn, M.C. Aronson The antiferromagnetic (AF) intermetallic compound Yb$_{3}$Pt$_{4}$ shows a magnetic phase diagram which includes a quantum critical point, but is different from other Yb-containing quantum critical compounds. We elucidated the zero-field behavior by neutron scattering on both polycrystal and single-crystal samples. The magnetic structure due to the single-site-low-symmetry Yb moments was determined by diffraction. The AF unit cell coincides with the crystallographic unit cell, and shows pairs of Yb nearest-neighbor moments pointing directly towards each other. The order parameter is consistent with a continuous transition at the N\'{e}el temperature (2.4 K) and can be described by a simple mean-field model. The ordered moment amounts to $\sim $ 1.2 $\mu _{B}$/Yb at 0 K. Inelastic neutron scattering reveals that the crystal electric field lifts the degeneracy of the Yb 4f ground state into 4 doublets, consistent with specific heat results. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700